I'm new to being DM and I'm trying to wrap my head around the mechanics of travelling through treacherous terrain. I have an example scenario in my head and it would be very helpful if someone could walk me through how they would go about it.
My party is going to try to cross a river in the middle of the night on a small rowboat. There are creatures lurking in the water.
As they start the crossing, I would describe the murky cold water and a sense that something isn't right. I'd like to roll for the boat randomly getting attacked by something in the water. I'm thinking they should have a chance to notice if a creature approaches the boat and either brace themselves or preemptively attack it. Is this passive or active? How do the rolls go for that (what abilities, what dice)? I'd prefer for an advantage to be gained by a PC noticing the incomimg creature. If they don't 'see' it, then the boat suddenly either gets rocked or smashed (randomly, how do I roll that?) If it gets rocked, then I'd like to roll for them to check if they fell out (what ability is that? How does it change if they see it coming and brace themselves first?) If it gets smashed, then they are now going to choose to swim or fight from the water, possibly different choices for each character. If some characters swim and others face the creature, how do I decide what the creature does? If the creature is chasing someone, how do I roll for the swimming race? If a player fights the creature while swimming, I'd imagine that's a tactical disadvantage against a creature that's at home in the water, how do I calculate that?
I realize this is a lot of a questions, but as I imagined this seemingly simple scenario I realized how difficult it is to do it fairly. I just want to have an idea so I can use it for this scenario but I think knowing how to go about this would cover a lot of situations in general.
So, as I normally say Passive Perception is more of an optional rule, if you don't want to use it don't. However, in this case if they just get in the boat without looking around or looking out for anything than it probably would be a typical example of Passive Perception. They could do lots of things after noticing it. If the monster attacks the boat, but not the players directly then the players would probably use an Acrobatics Check in order to remain balanced and in the boat. If the players Brace then they should get Advantage on that Acrobatics Check, and maybe use Strength instead of Dexterity if they want. As for whether the boat would be smashed you could either use a Damage Roll against the Boat, or a Strength Check (whether you add Proficiency or not is up to you). The party might also attempt to move the boat out of the way, use a Vehicles (Water) Tool Check with Dexterity as the Ability Score against the creature's Attack Roll. The party could also attempt to scare off the creature with Intimidation or Damage. A player could Prepare an Action to either stop the creature's attack by attempting a Grapple Check or attack them when in range. If they are in the water and attempt to flee by swimming you would have an Athletics Check versus an Athletics Check if the creature chases them. If the creature is a natural swimmer you might want to give it Advantage on that Athletics Check. The primary advantage the creature has is that water is its natural environment, it can move quickly in a 3-Dimensional Space. A Melee Attack shouldn't really have any Disadvantage in water, though if you feel the need to Disadvantage it, you might want to remove the Strength/Dexterity Bonus to Damage. A Ranged Attack on the other hand should have Disadvantage to Attack Rolls because of how the physics of Projectiles in water work in real life. Magic Attack Rolls should probably be left as is, though certain spells may act differently underwater. Fire Spells whiffing, Electric Spells hitting everyone automatically, etc. As for what the creature does, it would likely go for whatever seems like the easiest target for it. So if one player starts fighting it and the others flee, I would probably have it attack that player or attempt to drag them down to drown.
Other DMs can correct me if I'm wrong, but this is how I understand some of what you're asking.
Passive vs. active perception. In a situation like this you're going to roll a stealth check for the attackers. This is contested against either the passive perception or a perception check. What I've seen (I'm fairly new to DMing but I've been reading and watching stuff) is that DMs tend to use passive perception as a means of guaging whether the party might spot something when they're not actively looking for it. So if no one in your party states they're actively keeping watch for anything, you can use passive to judge if they spot or sense anything. Anyone with a passive perception higher than the creatures' stealth roll are going to get a sense of something amiss. The gap will help you judge how much they sense. If they are actively keeping watch, you'd have them roll a perception check. This would also be against the attacking creatures' stealth roll.
Checking for falling out of the boat is a Dexterity Saving Throw. Depending on the PCs chosen races/classes you may at your discretion deem it would make more sense to have them make a Strength Saving Throw. (i.e. if you feel in the situation that checking how well they keep a grip on the sides of the boat is more logical than seeing how well they keep their balance in the boat. It's going to depend on what the player actions are in the moment. It'll be a judgment call you make at the table.)
As to what dice, all checks and saves are a d20.
Let the dice decide who the creature attacks if the players end up in the water. When I need to make a random choice of which PC is attacked, I take the appropriate die that represents the number of PCs and assign each PC a number. There are 4 PCs in the group so it's usually a d4. Whichever number it lands on is which PC gets attacked. If there are only 2 choices for a creature to attack, I roll a d6 and have the PCs call evens or odds. Letting the dice decide keeps it fair and random. Sometimes, though, random isn't logical. Sometimes more intelligent creatures guage which PC is the biggest threat and target them. Less intelligent creatures might ignore PCs that haven't attacked it yet in favor of ones that have.
There are chase rules in the DMG. A chase is a skill challenge. Swimming will mean you have a lot of constitution and/or strength checks (again whichever makes sense in the moment at the table). It's your call to decide which ability check makes the most sense at the time. Generally, anything to do with swimming will be Strength unless they're underwater and trying to hold their breath. If that's the case you'll need to refer to the PHB. Chapter 8 talks about suffocating, which is related to CON score. Chapter 9 has rules for underwater combat which may also be helpful for fighting while swimming.
Read the combat rules in the PHB regarding advantage and disadvantage, too. They work slightly differently underwater and you may need to adapt those rules to fighting while trying to stay above the water's surface.
I'm pretty sure that cosplaywrite is also correct, though I wouldn't use Saves unless there is no better option. You can use a Dexterity Save or Check to avoid falling into the water to be sure, but as it says in the book under the examples of what Acrobatics can be used for: "stay upright on a rocking ship's deck". Sounds like that's basically what's happening. Also for Swimming you could use a Strength Save or Check, but once again under the examples of what Athletics can be used for: "swim or stay afloat in treacherous currents, storm-tossed waves, or areas of thick seaweed. Or another creature tries to push or pull you underwater or otherwise interfere with your swimming." While generic swimming may not require an Athletics Skill, during a chase it would make sense.
I contend that if the situation, reaction, or action can logically use a Skill or Tool, it should as those are the most special things about the characters outside of personality and history, the built in mechanics that make them individuals. And yes, all Checks and Saves are d20s.
Spotting the creature: there are a lot of factors to consider here. Since it is night, depends on amount of light, but under water in less than bright light would probably be total obscurity which is an auto fail for perception based on sight.
Whether the boat is rocked or destroyed: give the boat HP.
Whether PCs get knocked off rocking boat, Acrobatics check vs attack roll sounds good.
Swimming speeds are half walking speed unless otherwise stated.
Wow guys! These are some great thorough answers. Thanks especially to cosplaywrite for taking the time to address each piece of this in detail. With the varying opinions and options on each thing I think I'll be able to take this and wrap it into my own style. I'm feeling a lot more confident about this now. Thanks again everyone!
I'm pretty sure that cosplaywrite is also correct, though I wouldn't use Saves unless there is no better option. You can use a Dexterity Save or Check to avoid falling into the water to be sure, but as it says in the book under the examples of what Acrobatics can be used for: "stay upright on a rocking ship's deck". Sounds like that's basically what's happening. Also for Swimming you could use a Strength Save or Check, but once again under the examples of what Athletics can be used for: "swim or stay afloat in treacherous currents, storm-tossed waves, or areas of thick seaweed. Or another creature tries to push or pull you underwater or otherwise interfere with your swimming." While generic swimming may not require an Athletics Skill, during a chase it would make sense.
I contend that if the situation, reaction, or action can logically use a Skill or Tool, it should as those are the most special things about the characters outside of personality and history, the built in mechanics that make them individuals. And yes, all Checks and Saves are d20s.
Thank you for the correction on save vs. straight check. I'm still learning some of the nuances there.
The other answers seemed to have covered the majority of the points you asked about, but here are two thoughts I had that weren't addressed.
First, you had mentioned rolling randomly for something to attack. I would design a table for random encounters, possibly two tables if I wanted a random creature instead of a fixed creature. Let's say a d20 with 1-2 a creature attacks from below the boat (no perception available outside of magical options like detect evil and good, detect magic, or the like for appropriate creatures), 3-4 a creature attacks from the side (perception checks apply, no vehicle skill checks available due to speed of attack), 5-6 a creature attacks from the side (perception checks apply, vehicle skill for evade), 7-10 nothing happens, 11-13 creature begins circling boat, 14-17 creature not visible, but some proof of its presence, 18-20 creature takes some ranged attack (if available, otherwise nothing). The second table could be for the type of creature if you wanted that to be random (could include or simply determine the number). Additional tables could be made for whatever randomness you wish to include, say weather, happenings on the bank, random checks for sea sickness with a passive constitution check and immunity for sailors or others with backgrounds that would suggest no problems there. Then just roll a die for each corresponding table (each differentiated to address a certain table) for the specifics.
As for deciding whom the creature would attack, are there any details about the creature that would make it close one over the others? Perhaps it's hungry and the halfling looks snack size. Perhaps the 250 lb barbarian looks soft and large enough to last a few meals. The fighter is in plate and experience had shown that look promises disappointment for biting through, or that the weight of the armor slows the wearer down enough that it becomes simple to grapple it and drag it down to drown. Figure out why the creature is attacking and that will help you determine who to attack. Is it's nest/young close by and would the creature care? Determine who is closest and attack. Is the attack purely territorial? Determine the territory and then the closest within the territory or random. How intelligent is the creature and how wise? If either score suggests ability for tactics figure out the tactics ahead of time and perhaps develop a dragon age style priority list. Understand the creature and the fight will make more sense.
Thanks for the suggestions. I like your strategy of rolling the random attack.
This idea of getting inside the mind of the creature and considering its motivations is something I should definitely consider more. Even if the players don't know exactly why it happened that way, I feel like it's more satisfying as a DM to have a rationale behind what is going on. It would take a but more effort but I think the benefit is worth it, knowing that I'm not just picking things at random or like because a certain character was closest.
I think it's also easier to handle the encounter if you've put the work into it beforehand, assuming that you aren't going to have the creature just find the lightest armored, closest or whatever rationale that you want to have. It might also indicate when the creature would logically try to retreat, if at all (which most creatures would given the right criteria and reasonably developed sense of self preservation). I was actually thinking about this today and was contemplating doing something like 25% of starting HP would be a trigger to retreat as well as taking a hit that was over 50% of either starting HP or Max HP (depending on how I wanted to run the encounter). I've spent too long playing RPG video games and being pissed when creatures ran away, and that has colored my view on NPC retreating as a DM. Besides, it occurred to me that the reasoning behind why a creature/enemy wouldn't retreat could be a great story telling device for the PCs; "You could see in that half-elf's eyes that he knew he was going to die by remaining engaged with us. Why didn't he try to run away? What could possibly be worse than dying?" If this thought is considered by the players, then it might help build up the anticipation for the boss. But having no one run away means that this isn't even considered.
One of the five cardinal rules of being a PC, I believe. Its a bit "meta" as they say, but even greenhorn players may be able to figure out why there is a river crossing at that point in the adventure. Why else would there be a boat crossing? They may just seek a way to circumvent the encounter. Unless you have very kind players who know how to play along even when one sucks at being a DM, as we all do from time to time, they may become extraordinarily miffed when they loose a party member or two to a situation like that. Besides from being cliché it is an extremely dangerous encounter because non-aquatic beings are at a huge disadvantage when fighting aquatic ones while swimming or in a tiny boat (even if you DM the rules as-is and liberally). A giant squid, for example could just drag air breathing beings deep under underwater and wait for them to start drowning.
I admit to having an encounter with an aquatic being that lived under the ice in a lake that the PCs were crossing on foot. The players immediately started quipping about this "I bet there's a monster under the lake" and so on; I realized it would be simply silly from a story viewpoint if the monster did attack them while they were crossing. Not as much of a stereotype as the river crossing by boat/lurking monster perhaps, but still. Not my brightest moment.
I would try to make a twist on this instead, like perhaps they are on a crowded toll bridge manned by lazy and corrupt guards. The bridge is suddenly attacked by an aquatic monster while the PCs are waiting for their turn or perhaps even while they are in an argument with the guards.
1) There is either something in the water or there isn't. You decide. You don't need to make it random. You can still have the players roll dice to build tension but they will never know whether there was something there or not. The perception can be more important to the mood than the reality.
2) If you decide that there is something in the water. You will likely need a stat block for it. AC, HPs, attacks ... can it reach out of the water, is it attacking the boat, its occupants or both, is it intelligent? If the players take precautions to be quiet, whatever is in the water might not notice them.
3) If you are outside at night ... it is DARK. A full moon might provide dim light but it is otherwise considered dark. "Darkness creates a heavily obscured area. Characters face darkness outdoors at night (even most moonlit nights)". In darkness, a character without darkvision can't see effectively. Even a character with darkvision has disadvantage on perception checks. "In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.". Ddarkvision turns darkness to dim light which is lightly obscured and results in disadvantage on perception checks. This is -5 to passive perception. On the other hand, a warlock with devils sight can see in darkness as if it was daylight and does not suffer disadvantage.
4) Your situation has several checks ...
a) The players may make a stealth check against the passive perception of whatever is in the water to see if they can move the boat unnoticed
b) The creature in the water may make a stealth check against the players passive perception (keep in mind that passive can be used EVEN if the players are actively looking). Passive can be used as a floor value in the sense that the character should not have a perception result lower than their passive value. (Sage advice suggestion). Unless the characters have devils sight or a light source then the perception checks will be at disadvantage.
c) If the characters notice the approach of the creature they can respond. They could attack it. Some weapons may be adversely affected by the water. Ranged weapons would likely have disadvantage to hit a target underwater.
d) The creature attacks the boat trying to knock it over or it could attack creatures in the boat if it can reach them. Roll to hit. Perhaps do some damage. Allow the players to make either an Acrobatics or Athletics check (their choice) to avoid falling in the water unless they took precautions like tying themselves into the boat with a quick release knot which might give them advantage on the check. Acrobatics reflects keeping your balance while athletics could be your ability to hold on.
e) If they fall in the water then they have to swim. Underwater combat rules are on page 198 PHB. Athletics is used for swimming checks "Athletics. Your Strength (Athletics) check covers difficult situations you encounter while climbing, jumping, or swimming." though as a DM you can change skills or related stat for the check as you see fit. "While climbing or swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a climbing or swimming speed." Other than that 5e contains no additional rules for swimming in armor, carrying a pack, or anything else up to whatever rules you are using for encumbrance. So RAW, they can swim across the river with armor and full pack at a rate of 15'/turn (if they don't have a swim speed and the water is calm - normal terrain) .. rough water might be difficult terrain so they could try to swim 10'/turn. Combat could take place in the water but the players will have some difficulty dealing with the attackers since they will have disadvantage on their attacks unless they have a swim speed.
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Hi,
I'm new to being DM and I'm trying to wrap my head around the mechanics of travelling through treacherous terrain. I have an example scenario in my head and it would be very helpful if someone could walk me through how they would go about it.
My party is going to try to cross a river in the middle of the night on a small rowboat. There are creatures lurking in the water.
As they start the crossing, I would describe the murky cold water and a sense that something isn't right. I'd like to roll for the boat randomly getting attacked by something in the water. I'm thinking they should have a chance to notice if a creature approaches the boat and either brace themselves or preemptively attack it. Is this passive or active? How do the rolls go for that (what abilities, what dice)? I'd prefer for an advantage to be gained by a PC noticing the incomimg creature. If they don't 'see' it, then the boat suddenly either gets rocked or smashed (randomly, how do I roll that?) If it gets rocked, then I'd like to roll for them to check if they fell out (what ability is that? How does it change if they see it coming and brace themselves first?) If it gets smashed, then they are now going to choose to swim or fight from the water, possibly different choices for each character. If some characters swim and others face the creature, how do I decide what the creature does? If the creature is chasing someone, how do I roll for the swimming race? If a player fights the creature while swimming, I'd imagine that's a tactical disadvantage against a creature that's at home in the water, how do I calculate that?
I realize this is a lot of a questions, but as I imagined this seemingly simple scenario I realized how difficult it is to do it fairly. I just want to have an idea so I can use it for this scenario but I think knowing how to go about this would cover a lot of situations in general.
Thanks in advance for your help!
So, as I normally say Passive Perception is more of an optional rule, if you don't want to use it don't. However, in this case if they just get in the boat without looking around or looking out for anything than it probably would be a typical example of Passive Perception. They could do lots of things after noticing it. If the monster attacks the boat, but not the players directly then the players would probably use an Acrobatics Check in order to remain balanced and in the boat. If the players Brace then they should get Advantage on that Acrobatics Check, and maybe use Strength instead of Dexterity if they want. As for whether the boat would be smashed you could either use a Damage Roll against the Boat, or a Strength Check (whether you add Proficiency or not is up to you). The party might also attempt to move the boat out of the way, use a Vehicles (Water) Tool Check with Dexterity as the Ability Score against the creature's Attack Roll. The party could also attempt to scare off the creature with Intimidation or Damage. A player could Prepare an Action to either stop the creature's attack by attempting a Grapple Check or attack them when in range. If they are in the water and attempt to flee by swimming you would have an Athletics Check versus an Athletics Check if the creature chases them. If the creature is a natural swimmer you might want to give it Advantage on that Athletics Check. The primary advantage the creature has is that water is its natural environment, it can move quickly in a 3-Dimensional Space. A Melee Attack shouldn't really have any Disadvantage in water, though if you feel the need to Disadvantage it, you might want to remove the Strength/Dexterity Bonus to Damage. A Ranged Attack on the other hand should have Disadvantage to Attack Rolls because of how the physics of Projectiles in water work in real life. Magic Attack Rolls should probably be left as is, though certain spells may act differently underwater. Fire Spells whiffing, Electric Spells hitting everyone automatically, etc. As for what the creature does, it would likely go for whatever seems like the easiest target for it. So if one player starts fighting it and the others flee, I would probably have it attack that player or attempt to drag them down to drown.
Other DMs can correct me if I'm wrong, but this is how I understand some of what you're asking.
Passive vs. active perception. In a situation like this you're going to roll a stealth check for the attackers. This is contested against either the passive perception or a perception check. What I've seen (I'm fairly new to DMing but I've been reading and watching stuff) is that DMs tend to use passive perception as a means of guaging whether the party might spot something when they're not actively looking for it. So if no one in your party states they're actively keeping watch for anything, you can use passive to judge if they spot or sense anything. Anyone with a passive perception higher than the creatures' stealth roll are going to get a sense of something amiss. The gap will help you judge how much they sense. If they are actively keeping watch, you'd have them roll a perception check. This would also be against the attacking creatures' stealth roll.
Checking for falling out of the boat is a Dexterity Saving Throw. Depending on the PCs chosen races/classes you may at your discretion deem it would make more sense to have them make a Strength Saving Throw. (i.e. if you feel in the situation that checking how well they keep a grip on the sides of the boat is more logical than seeing how well they keep their balance in the boat. It's going to depend on what the player actions are in the moment. It'll be a judgment call you make at the table.)
As to what dice, all checks and saves are a d20.
Let the dice decide who the creature attacks if the players end up in the water. When I need to make a random choice of which PC is attacked, I take the appropriate die that represents the number of PCs and assign each PC a number. There are 4 PCs in the group so it's usually a d4. Whichever number it lands on is which PC gets attacked. If there are only 2 choices for a creature to attack, I roll a d6 and have the PCs call evens or odds. Letting the dice decide keeps it fair and random. Sometimes, though, random isn't logical. Sometimes more intelligent creatures guage which PC is the biggest threat and target them. Less intelligent creatures might ignore PCs that haven't attacked it yet in favor of ones that have.
There are chase rules in the DMG. A chase is a skill challenge. Swimming will mean you have a lot of constitution and/or strength checks (again whichever makes sense in the moment at the table). It's your call to decide which ability check makes the most sense at the time. Generally, anything to do with swimming will be Strength unless they're underwater and trying to hold their breath. If that's the case you'll need to refer to the PHB. Chapter 8 talks about suffocating, which is related to CON score. Chapter 9 has rules for underwater combat which may also be helpful for fighting while swimming.
Read the combat rules in the PHB regarding advantage and disadvantage, too. They work slightly differently underwater and you may need to adapt those rules to fighting while trying to stay above the water's surface.
My Homebrew Backgrounds | Feats | Magic Items | Monsters | Races | Subclasses
I'm pretty sure that cosplaywrite is also correct, though I wouldn't use Saves unless there is no better option. You can use a Dexterity Save or Check to avoid falling into the water to be sure, but as it says in the book under the examples of what Acrobatics can be used for: "stay upright on a rocking ship's deck". Sounds like that's basically what's happening. Also for Swimming you could use a Strength Save or Check, but once again under the examples of what Athletics can be used for: "swim or stay afloat in treacherous currents, storm-tossed waves, or areas of thick seaweed. Or another creature tries to push or pull you underwater or otherwise interfere with your swimming." While generic swimming may not require an Athletics Skill, during a chase it would make sense.
I contend that if the situation, reaction, or action can logically use a Skill or Tool, it should as those are the most special things about the characters outside of personality and history, the built in mechanics that make them individuals. And yes, all Checks and Saves are d20s.
In parts:
Spotting the creature: there are a lot of factors to consider here. Since it is night, depends on amount of light, but under water in less than bright light would probably be total obscurity which is an auto fail for perception based on sight.
Whether the boat is rocked or destroyed: give the boat HP.
Whether PCs get knocked off rocking boat, Acrobatics check vs attack roll sounds good.
Swimming speeds are half walking speed unless otherwise stated.
Use chase rules. https://www.dndbeyond.com/compendium/rules/dmg/running-the-game#ChasesThe chase rules are not designed for short chases through water, use water combat rules instead. https://www.dndbeyond.com/compendium/rules/basic-rules/combat#UnderwaterCombat Wow guys! These are some great thorough answers. Thanks especially to cosplaywrite for taking the time to address each piece of this in detail. With the varying opinions and options on each thing I think I'll be able to take this and wrap it into my own style. I'm feeling a lot more confident about this now. Thanks again everyone!
Thank you for the correction on save vs. straight check. I'm still learning some of the nuances there.
My Homebrew Backgrounds | Feats | Magic Items | Monsters | Races | Subclasses
The other answers seemed to have covered the majority of the points you asked about, but here are two thoughts I had that weren't addressed.
First, you had mentioned rolling randomly for something to attack. I would design a table for random encounters, possibly two tables if I wanted a random creature instead of a fixed creature. Let's say a d20 with 1-2 a creature attacks from below the boat (no perception available outside of magical options like detect evil and good, detect magic, or the like for appropriate creatures), 3-4 a creature attacks from the side (perception checks apply, no vehicle skill checks available due to speed of attack), 5-6 a creature attacks from the side (perception checks apply, vehicle skill for evade), 7-10 nothing happens, 11-13 creature begins circling boat, 14-17 creature not visible, but some proof of its presence, 18-20 creature takes some ranged attack (if available, otherwise nothing). The second table could be for the type of creature if you wanted that to be random (could include or simply determine the number). Additional tables could be made for whatever randomness you wish to include, say weather, happenings on the bank, random checks for sea sickness with a passive constitution check and immunity for sailors or others with backgrounds that would suggest no problems there. Then just roll a die for each corresponding table (each differentiated to address a certain table) for the specifics.
As for deciding whom the creature would attack, are there any details about the creature that would make it close one over the others? Perhaps it's hungry and the halfling looks snack size. Perhaps the 250 lb barbarian looks soft and large enough to last a few meals. The fighter is in plate and experience had shown that look promises disappointment for biting through, or that the weight of the armor slows the wearer down enough that it becomes simple to grapple it and drag it down to drown. Figure out why the creature is attacking and that will help you determine who to attack. Is it's nest/young close by and would the creature care? Determine who is closest and attack. Is the attack purely territorial? Determine the territory and then the closest within the territory or random. How intelligent is the creature and how wise? If either score suggests ability for tactics figure out the tactics ahead of time and perhaps develop a dragon age style priority list. Understand the creature and the fight will make more sense.
Thanks for the suggestions. I like your strategy of rolling the random attack.
This idea of getting inside the mind of the creature and considering its motivations is something I should definitely consider more. Even if the players don't know exactly why it happened that way, I feel like it's more satisfying as a DM to have a rationale behind what is going on. It would take a but more effort but I think the benefit is worth it, knowing that I'm not just picking things at random or like because a certain character was closest.
Great feedback, thanks again!
I think it's also easier to handle the encounter if you've put the work into it beforehand, assuming that you aren't going to have the creature just find the lightest armored, closest or whatever rationale that you want to have. It might also indicate when the creature would logically try to retreat, if at all (which most creatures would given the right criteria and reasonably developed sense of self preservation). I was actually thinking about this today and was contemplating doing something like 25% of starting HP would be a trigger to retreat as well as taking a hit that was over 50% of either starting HP or Max HP (depending on how I wanted to run the encounter). I've spent too long playing RPG video games and being pissed when creatures ran away, and that has colored my view on NPC retreating as a DM. Besides, it occurred to me that the reasoning behind why a creature/enemy wouldn't retreat could be a great story telling device for the PCs; "You could see in that half-elf's eyes that he knew he was going to die by remaining engaged with us. Why didn't he try to run away? What could possibly be worse than dying?" If this thought is considered by the players, then it might help build up the anticipation for the boss. But having no one run away means that this isn't even considered.
Do not get in the boat!
One of the five cardinal rules of being a PC, I believe. Its a bit "meta" as they say, but even greenhorn players may be able to figure out why there is a river crossing at that point in the adventure. Why else would there be a boat crossing? They may just seek a way to circumvent the encounter. Unless you have very kind players who know how to play along even when one sucks at being a DM, as we all do from time to time, they may become extraordinarily miffed when they loose a party member or two to a situation like that. Besides from being cliché it is an extremely dangerous encounter because non-aquatic beings are at a huge disadvantage when fighting aquatic ones while swimming or in a tiny boat (even if you DM the rules as-is and liberally). A giant squid, for example could just drag air breathing beings deep under underwater and wait for them to start drowning.
I admit to having an encounter with an aquatic being that lived under the ice in a lake that the PCs were crossing on foot. The players immediately started quipping about this "I bet there's a monster under the lake" and so on; I realized it would be simply silly from a story viewpoint if the monster did attack them while they were crossing. Not as much of a stereotype as the river crossing by boat/lurking monster perhaps, but still. Not my brightest moment.
I would try to make a twist on this instead, like perhaps they are on a crowded toll bridge manned by lazy and corrupt guards. The bridge is suddenly attacked by an aquatic monster while the PCs are waiting for their turn or perhaps even while they are in an argument with the guards.
Just a couple of additional thoughts ..
1) There is either something in the water or there isn't. You decide. You don't need to make it random. You can still have the players roll dice to build tension but they will never know whether there was something there or not. The perception can be more important to the mood than the reality.
2) If you decide that there is something in the water. You will likely need a stat block for it. AC, HPs, attacks ... can it reach out of the water, is it attacking the boat, its occupants or both, is it intelligent? If the players take precautions to be quiet, whatever is in the water might not notice them.
3) If you are outside at night ... it is DARK. A full moon might provide dim light but it is otherwise considered dark. "Darkness creates a heavily obscured area. Characters
face darkness outdoors at night (even most moonlit nights)". In darkness, a character without darkvision can't see effectively. Even a character with darkvision has disadvantage on perception checks. "In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.". Ddarkvision turns darkness to dim light which is lightly obscured and results in disadvantage on perception checks. This is -5 to passive perception. On the other hand, a warlock with devils sight can see in darkness as if it was daylight and does not suffer disadvantage.
4) Your situation has several checks ...
a) The players may make a stealth check against the passive perception of whatever is in the water to see if they can move the boat unnoticed
b) The creature in the water may make a stealth check against the players passive perception (keep in mind that passive can be used EVEN if the players are actively looking). Passive can be used as a floor value in the sense that the character should not have a perception result lower than their passive value. (Sage advice suggestion). Unless the characters have devils sight or a light source then the perception checks will be at disadvantage.
c) If the characters notice the approach of the creature they can respond. They could attack it. Some weapons may be adversely affected by the water. Ranged weapons would likely have disadvantage to hit a target underwater.
d) The creature attacks the boat trying to knock it over or it could attack creatures in the boat if it can reach them. Roll to hit. Perhaps do some damage. Allow the players to make either an Acrobatics or Athletics check (their choice) to avoid falling in the water unless they took precautions like tying themselves into the boat with a quick release knot which might give them advantage on the check. Acrobatics reflects keeping your balance while athletics could be your ability to hold on.
e) If they fall in the water then they have to swim. Underwater combat rules are on page 198 PHB. Athletics is used for swimming checks "Athletics. Your Strength (Athletics) check covers difficult situations you encounter while climbing, jumping, or swimming." though as a DM you can change skills or related stat for the check as you see fit. "While climbing or swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a climbing or swimming speed." Other than that 5e contains no additional rules for swimming in armor, carrying a pack, or anything else up to whatever rules you are using for encumbrance. So RAW, they can swim across the river with armor and full pack at a rate of 15'/turn (if they don't have a swim speed and the water is calm - normal terrain) .. rough water might be difficult terrain so they could try to swim 10'/turn. Combat could take place in the water but the players will have some difficulty dealing with the attackers since they will have disadvantage on their attacks unless they have a swim speed.