You see, it happens that i'm going to be the DM of a new campaign. It happens that the intro and first levels will be based in a coastal/underwater adventure. I've found rules about fighting underwater and drowning, and i'm a bit confused about the times that the characters can hold their breath, no matter what they are doing underwater.
By RAW, a character can hold his/her breath by a numer of mins equal to 1+cons MOD. (PHB, 183, suffocating). And also by RAW, fighting underwater does not say that recieving hits or moving violently underwater shortens this time (PHB, 198).
So, my question here, after looking for some extra information in the DM's guide is...Is it any rule over there that extends this kind of thing?. I mean, by this rule, a character with 12 CONS can be fighting underwater for 20 rounds without any issues...how many combats have you had that lasted 20 rounds? It kinda blows away the threat that beign underwater should impose over the players, this times are just long enough for them to fight as good as a fish if they have the correct weapons. Damn, they can even fight with ranged atacks if they have a crossbow.
I have a group that takes very, VERY bad every single homebrew rule i proppose that implies taking something from the group that the RAW rules give them (nerfing), but in this case i just cannot feel right thinking that an underwater combat can turn from suffocating and exasperating to "meh, we have 30 rounds to spare killing this mob, lets just swim around and blow up the besterd".
Remember that movement is 1/2 in water - so their normal 300' range in that minute is now 150', which is still a good distance. Put in some difficult terrain ( kelp forest? ), and they're down to 1/3 or 100' range ( 1' extra movement cost for water, 1' extra movement cost for terrain, for every 1' travel ). Underwater Creatures with a swim speed don't suffer the extra cost for water, and can swim rings around them.
You're assuming that the combat starts the moment the Characters hit the water. This might be the case. It may also be the case where they need to swim down to the combat area. This cuts into time. If that 12 Con Character takes a minute to swim down to the combat, they're in trouble, as they now need to immediately turn around again.
Is there any current? You'd be surprised how quickly that adds up. 10' per round = 60' per minute = 0.7Mph, where river currents range from 0 to 7mph. A medium river current of 2 mph is 30'+ per round in the direction of the current, and the Characters are moving at 1/2 speed. They have to dash to stand still, while the Sahuagins calmly tread water to stay in place and chuck spears into them.
I don't think that Characters have this time reduced for moving violently underwater, because Characters can't move violently underwater. This is why anything not a " dagger, javelin, short-sword, spear, or trident" has disadvantage on the attack roll. Even a heavy crossbow only has a range of 100' With their superior movement, clever swimming opponents can stay out of this range easily. An opponent with a good ranged attack and a decent swim speed can zip in, fire, and zip out of range again. Heck, give those Sahuagin crossbows and the Party is in for a world of hurt. They can shred the Party at range, and never stay in range of the Party to get hit ( although, held attacks from the Party will snipe them back - still the engagement is on the Sahuagin 's terms ).
Spells. There's a number of Jeremy Crawford "Sage Advice" tweets ( here, here, andhere ) about underwater spellcasting. I interpret these as meaning there's nothing preventing you from casting a verbal component spell at all, but you stop holding your breath, and start drowning when you do, RAW. A nice DM might just take off a set number of rounds that the Character can hold their breath ( 5? 10? ). That's not nerfing your Players, that's cutting them some slack.
Put it all together, and it doesn't seem like your Party can be saying "meh, we have 30 rounds to spare killing this mob, lets just swim around and blow up the besterd", at all: No spells ( unless your casters like drowning), 1/2 movement, disadvantage on most attacks, currents can be deadly, and an inability to travel that far from a nice pocket of air.
Unless they're fighting in a swimming pool, I think this is still plenty challenging, even if they're not in immediate danger of drowning.
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By RAW, a character can hold his/her breath by a numer of mins equal to 1+cons MOD. (PHB, 183, suffocating). And also by RAW, fighting underwater does not say that recieving hits or moving violently underwater shortens this time (PHB, 198).
So, my question here, after looking for some extra information in the DM's guide is...Is it any rule over there that extends this kind of thing?. I mean, by this rule, a character with 12 CONS can be fighting underwater for 20 rounds without any issues...how many combats have you had that lasted 20 rounds? It kinda blows away the threat that beign underwater should impose over the players, this times are just long enough for them to fight as good as a fish if they have the correct weapons. Damn, they can even fight with ranged atacks if they have a crossbow.
Your math here is off: A 12 CON = +1 modifier, which gives the character a total of 2minutes underwater. Then, after that, they have 1round to reach some method with which to breath before they fall unconscious and begin to drown. In total that gives a character, with 12 Con, 21rounds with which to explore, engage in combat, and otherwise navigate underwater. Take into account movement reduction, limited weapon capabilities, inability to cast many spells, as well as vision impairment, and you have a very difficult time for your players. While the numbers may seem like they're absurdly large, there are many things working against the players that make the journey underwater very difficult.
As to making it so that certain activities may affect how long a character can hold their breath, this is something that you can do with a little grace. Activities like trying to move heavy objects, combat, escape rip tides, or swim against a strong current could impose a CON save, use a minor DC like 10 and remove 1 round. It is a minor inconvenience at first, but it can add up if they're not paying attention. A simple explanation of "Your extra effort taxes your lungs and you feel them begin to burn, you lose 1 round from holding your breath".
Most players won't argue a small change like that, and those that do should be reminded that you are the DM. The book is not the final say when it comes to the rules, you are.
Set a damage quantity to correspond to 1 round of air lost, scaled to whatever the party's level is. When people are hit by attacks, their natural response would be to scream/yelp/etc. and accidentally lose some air. Seems fair!
I recently used a houseruled "breath point" system to track this, where 1 breath point equals 1 second of underwater time. So, for example, a 12 constitution character gets 120 breath points. Breath points are lost in the following ways:
-Time - Every round the character spends underwater uses 6 breath points
-Spell casting with a verbal component - A spell with a casting time of 1 action uses 6 breath points, a bonus action uses 3
-Taking damage - you lose breath points equal to any damage taken
This worked out pretty well. It wasn't any more cumbersome than just tracking the number of rounds under water, and took into account things that would make you lose your breath.
Remember that underwater is a three dimensional environment and the PCs may have to expend movement vertically as well as horizontally. Movement requires time. If their underwater foes fall back instead of advancing they use up a lot of their breath holding time just trying to get close enough for combat.
Twenty rounds under water does not seem so long if the characters have more to do in that time than just the clash of blows. Add a sunken ship or kelp forest for them to get tanged in and start a countdown timer. Underwater action when you can't breathe water can get scary in a hurry. be sure to start the "holding your breath" clock from the moment they stick their face under the surface.
I recently used a houseruled "breath point" system to track this, where 1 breath point equals 1 second of underwater time. So, for example, a 12 constitution character gets 120 breath points. Breath points are lost in the following ways:
-Time - Every round the character spends underwater uses 6 breath points
-Spell casting with a verbal component - A spell with a casting time of 1 action uses 6 breath points, a bonus action uses 3
-Taking damage - you lose breath points equal to any damage taken
This worked out pretty well. It wasn't any more cumbersome than just tracking the number of rounds under water, and took into account things that would make you lose your breath.
This seems like a very good idea, sadly, i cannot put some homebrew rule in my game that changes something beneficing the players, because they enrage. I'm aware this is not the best quirk for a group but, as we say in spain, one has to know the sheeps he keeps.
Remember that underwater is a three dimensional environment and the PCs may have to expend movement vertically as well as horizontally. Movement requires time. If their underwater foes fall back instead of advancing they use up a lot of their breath holding time just trying to get close enough for combat.
Twenty rounds under water does not seem so long if the characters have more to do in that time than just the clash of blows. Add a sunken ship or kelp forest for them to get tanged in and start a countdown timer. Underwater action when you can't breathe water can get scary in a hurry. be sure to start the "holding your breath" clock from the moment they stick their face under the surface.
This can work pretty well for a 20 round player submerged, and its completely true i should use the underwater environment to put pressure into the fight by adding dangers to the scenario. However, that's the amount of time that the wizard of my party can hold his breth, the rest of players, 4 in total, can hold it for a min of 30 rounds, even 40 rounds. Thats a lot time pursuing a foe, and even a lot of time to cut the weeds entangling them.
I dont want the underwater combat to kill them tho, i want it to make them feel like they are completely out of the comfort zone. Except for the triton in the party of course, that guy is obviously in his terrain. XD
I will try to apply these suggestions, they are all awsome.
Something else that might be possible to play with, is a series of encounters, underwater, where the Party is hop-scotching from one zone of safety to another. They start on the surface, and have to fight their way down to the underwater cave, which contains breathable air, from there, they have to swim through a series of fast current lava tubes until they get to the next pocket of air, etc.
This could give an interesting sensation of urgency ( they basically need to solve the next adventure barrier quickly, before they drown ), and a sensation that they are overextending themselves, as there is soon no way for them to quickly return to the surface and safety - they have to make their way back, through many of the same challenges ( or new challenges that have moved into the space of the old challenge they removed ).
You could - essentially - make a small classic Dungeon Crawl with a very interesting panicked urgency sub-aquatic atmosphere, and some serious time pressures ( solve the next barrier and get through it, before you drown! ).
This could be tied to any environment where exposure is quickly but not immediately lethal, as well - extreme heat, cave complexes filled with poison gas, vacuum of space ( if you have Spelljammers in your setting), etc. - although underwater ( and perhaps space ) would give you the interesting possibility of arranging your encounters three dimensionally.
Disclaimer: This signature is a badge of membership in the Forum Loudmouth Club. We are all friends. We are not attacking each other. We are engaging in spirited, friendly debate with one another. We may get snarky, but these are not attacks. Thank you for not reporting us.
Something else that might be possible to play with, is a series of encounters, underwater, where the Party is hop-scotching from one zone of safety to another. They start on the surface, and have to fight their way down to the underwater cave, which contains breathable air, from there, they have to swim through a series of fast current lava tubes until they get to the next pocket of air, etc.
This could give an interesting sensation of urgency ( they basically need to solve the next adventure barrier quickly, before they drown ), and a sensation that they are overextending themselves, as there is soon no way for them to quickly return to the surface and safety - they have to make their way back, through many of the same challenges ( or new challenges that have moved into the space of the old challenge they removed ).
You could - essentially - make a small classic Dungeon Crawl with a very interesting panicked urgency sub-aquatic atmosphere, and some serious time pressures ( solve the next barrier and get through it, before you drown! ).
This could be tied to any environment where exposure is quickly but not immediately lethal, as well - extreme heat, cave complexes filled with poison gas, vacuum of space ( if you have Spelljammers in your setting), etc. - although underwater ( and perhaps space ) would give you the interesting possibility of arranging your encounters three dimensionally.
I may have to file this one for my own use ;)
Now this is very interesting. My group is heavy melee/martial advanced, a challenge underwater that involves crawling, stretching, even disabling a drowned trap that stills works can be very, VERY interesting. Also the dungeon may be some interesting and original thing, like beign trapped in the last floor of a big sinking boat or an underwater cave in the ruins of a lost elvish civilization....even maybe a jail in the lands of an evil triton lord.
Thanks for sharing your ideas, i think i can build something very nice from this
I recently used a houseruled "breath point" system to track this, where 1 breath point equals 1 second of underwater time. So, for example, a 12 constitution character gets 120 breath points. Breath points are lost in the following ways:
-Time - Every round the character spends underwater uses 6 breath points
-Spell casting with a verbal component - A spell with a casting time of 1 action uses 6 breath points, a bonus action uses 3
-Taking damage - you lose breath points equal to any damage taken
This worked out pretty well. It wasn't any more cumbersome than just tracking the number of rounds under water, and took into account things that would make you lose your breath.
This seems like a very good idea, sadly, i cannot put some homebrew rule in my game that changes something beneficing the players, because they enrage. I'm aware this is not the best quirk for a group but, as we say in spain, one has to know the sheeps he keeps.
So tell them that if they want to play, they go by your rules! And that if they are that upset about the rules you put in place, they can leave but if they stay, your rule is law at the table, being the dungeon master.
From the Basic Rules/Player's Handbook section on Constitution Checks -
The DM might call for a Constitution check when you try to accomplish tasks like the following:
Hold your breath
I'll be sending my children into shark infested waters in a week or two, so am trying to get my head around this topic...
There's a good argument to be had that just because a creature can hold its breath for 1+Con Mod minutes, doesn't mean they will (especially if they're being attacked). The 1+Con Mod rule for holding your breath is from the "Suffocating" section of the PHB - arguably this is a higher limit, not an automatically given allowance to 'spend' before you suffocate. I'd assume a check similar to spell concentration would be appropriate for holding breath while taking damage... although it is a bit strange that it isn't mentioned one way or the other in the Underwater Combat rules.
Remember that the players can be grappled, netted and the deep warlock can use grasp of hadar to keep pulling them down. In fact, use a NPC guide that gets kidnapped this way to inspire terror. Surprise round NPC receives eldritch bolt and gets pulled 20ft into the water. Next round is hit again and pulled 20ft under where it is netted and grappled by multiple Sahaugin with tridents. by end of 1st active round npc is 40 ft away, surrounded by enemies and under at least 20 ft of water. 2nd round they leave with their food into the dark depths.
Vedexent put it really greatly. I ran an underwater arc at one point where I at least gave my party some diving gear that would break after three hits before somebody would have to start holding their breath. But there are certainly ways to make interesting engagements while helping the party consider the value of their oxygen and the limiters they might have by being underwater. The movement, the disadvantage, the limitations on capabilities, etc. are all far more difficult to work with in practice than you might initially expect- especially when the party likely is dealing with something that has an advantage underwater compared to them. I put the party against MTF's Leviathan at the bottom of the sea and BOY WAS THAT INTENSE! If you really want to push harder though, you can always as a DM choose to interpret rules how you would like. If you think that people should lose some oxygen when they get hit, tell the PC to roll a CON save as if they were concentrating and find a way to determine how many rounds of oxygen they lose by it. Underwater encounters are certainly a place for creating a great sense of isolation and urgency; if your party isn't packing easy outs like Water Breathing or Water Walk, make them feel the panic of being without a guarantee of survival!
I honestly wonder, who thinks drowning is fun? The threat of drowning is what makes things fun. With all the ways player characters can avoid it, you have to push very hard indeed to make them drown. Adventures under the water are a way to give people a new setting to interact with, it's not supposed to be something you kill their characters with directly, you do that with the monsters if you have to do it at all. I approach killing player characters as something I accept, because it happens sometimes, but I won't set out with the intention of doing it.
If you set out with the attitude that you're going to force your player's characters to fail by making a threat and then carrying it out, they are likely to find something better to do with their time sooner or later. Combat under water is fun. Drowning isn't.
Breathing is only one part of the equation. Most playable races don’t have a swim speed.
If they are going up against underwater fighters with blind fighting style, the enemy might churn up the water to blind their foes. This leaves the party without ranged attacks/spells and the enemy has advantage on attacks.
This actually works well on land too. Spellcaster casts “fog cloud” over area and a number of lvl 2 fighter/rogues with blind fighting engage in combat.
Undead that drag or push people into the water where a swarm of piranha wait.
Let the PC’s rescue the NPC only to have its water breathing dispelled, now they have to make the choice of rescuing the NPC or winning the fight.
Geann said it well. Drowning is not a fun mechanic to add to combat. It's a limitation on PCs that changes the equation on how to deal with a challenge. In short, you need to flee/avoid underwater combat unless you have the proper tools to handle it. For non-combat challenges, it can serve as a nice ticking clock.
Hi lads! thanks for entering my thread
You see, it happens that i'm going to be the DM of a new campaign. It happens that the intro and first levels will be based in a coastal/underwater adventure. I've found rules about fighting underwater and drowning, and i'm a bit confused about the times that the characters can hold their breath, no matter what they are doing underwater.
By RAW, a character can hold his/her breath by a numer of mins equal to 1+cons MOD. (PHB, 183, suffocating). And also by RAW, fighting underwater does not say that recieving hits or moving violently underwater shortens this time (PHB, 198).
So, my question here, after looking for some extra information in the DM's guide is...Is it any rule over there that extends this kind of thing?. I mean, by this rule, a character with 12 CONS can be fighting underwater for 20 rounds without any issues...how many combats have you had that lasted 20 rounds? It kinda blows away the threat that beign underwater should impose over the players, this times are just long enough for them to fight as good as a fish if they have the correct weapons. Damn, they can even fight with ranged atacks if they have a crossbow.
I have a group that takes very, VERY bad every single homebrew rule i proppose that implies taking something from the group that the RAW rules give them (nerfing), but in this case i just cannot feel right thinking that an underwater combat can turn from suffocating and exasperating to "meh, we have 30 rounds to spare killing this mob, lets just swim around and blow up the besterd".
Any ideas? Thanks in advance
I don't have answers just some thoughts.
Remember that movement is 1/2 in water - so their normal 300' range in that minute is now 150', which is still a good distance. Put in some difficult terrain ( kelp forest? ), and they're down to 1/3 or 100' range ( 1' extra movement cost for water, 1' extra movement cost for terrain, for every 1' travel ). Underwater Creatures with a swim speed don't suffer the extra cost for water, and can swim rings around them.
You're assuming that the combat starts the moment the Characters hit the water. This might be the case. It may also be the case where they need to swim down to the combat area. This cuts into time. If that 12 Con Character takes a minute to swim down to the combat, they're in trouble, as they now need to immediately turn around again.
Is there any current? You'd be surprised how quickly that adds up. 10' per round = 60' per minute = 0.7Mph, where river currents range from 0 to 7mph. A medium river current of 2 mph is 30'+ per round in the direction of the current, and the Characters are moving at 1/2 speed. They have to dash to stand still, while the Sahuagins calmly tread water to stay in place and chuck spears into them.
I don't think that Characters have this time reduced for moving violently underwater, because Characters can't move violently underwater. This is why anything not a " dagger, javelin, short-sword, spear, or trident" has disadvantage on the attack roll. Even a heavy crossbow only has a range of 100' With their superior movement, clever swimming opponents can stay out of this range easily. An opponent with a good ranged attack and a decent swim speed can zip in, fire, and zip out of range again. Heck, give those Sahuagin crossbows and the Party is in for a world of hurt. They can shred the Party at range, and never stay in range of the Party to get hit ( although, held attacks from the Party will snipe them back - still the engagement is on the Sahuagin 's terms ).
Spells. There's a number of Jeremy Crawford "Sage Advice" tweets ( here, here, and here ) about underwater spellcasting. I interpret these as meaning there's nothing preventing you from casting a verbal component spell at all, but you stop holding your breath, and start drowning when you do, RAW. A nice DM might just take off a set number of rounds that the Character can hold their breath ( 5? 10? ). That's not nerfing your Players, that's cutting them some slack.
Put it all together, and it doesn't seem like your Party can be saying "meh, we have 30 rounds to spare killing this mob, lets just swim around and blow up the besterd", at all: No spells ( unless your casters like drowning), 1/2 movement, disadvantage on most attacks, currents can be deadly, and an inability to travel that far from a nice pocket of air.
Unless they're fighting in a swimming pool, I think this is still plenty challenging, even if they're not in immediate danger of drowning.
My DM Philosophy, as summed up by other people: https://drive.google.com/file/d/1rN5w4-azTq3Kbn0Yvk9nfqQhwQ1R5by1/view
Disclaimer: This signature is a badge of membership in the Forum Loudmouth Club. We are all friends. We are not attacking each other. We are engaging in spirited, friendly debate with one another. We may get snarky, but these are not attacks. Thank you for not reporting us.
Your math here is off: A 12 CON = +1 modifier, which gives the character a total of 2 minutes underwater. Then, after that, they have 1 round to reach some method with which to breath before they fall unconscious and begin to drown. In total that gives a character, with 12 Con, 21 rounds with which to explore, engage in combat, and otherwise navigate underwater. Take into account movement reduction, limited weapon capabilities, inability to cast many spells, as well as vision impairment, and you have a very difficult time for your players. While the numbers may seem like they're absurdly large, there are many things working against the players that make the journey underwater very difficult.
As to making it so that certain activities may affect how long a character can hold their breath, this is something that you can do with a little grace. Activities like trying to move heavy objects, combat, escape rip tides, or swim against a strong current could impose a CON save, use a minor DC like 10 and remove 1 round. It is a minor inconvenience at first, but it can add up if they're not paying attention. A simple explanation of "Your extra effort taxes your lungs and you feel them begin to burn, you lose 1 round from holding your breath".
Most players won't argue a small change like that, and those that do should be reminded that you are the DM. The book is not the final say when it comes to the rules, you are.
Set a damage quantity to correspond to 1 round of air lost, scaled to whatever the party's level is. When people are hit by attacks, their natural response would be to scream/yelp/etc. and accidentally lose some air. Seems fair!
Thank you very much for the answers! You guys really helped me a ton!
I recently used a houseruled "breath point" system to track this, where 1 breath point equals 1 second of underwater time. So, for example, a 12 constitution character gets 120 breath points. Breath points are lost in the following ways:
-Time - Every round the character spends underwater uses 6 breath points
-Spell casting with a verbal component - A spell with a casting time of 1 action uses 6 breath points, a bonus action uses 3
-Taking damage - you lose breath points equal to any damage taken
This worked out pretty well. It wasn't any more cumbersome than just tracking the number of rounds under water, and took into account things that would make you lose your breath.
Remember that underwater is a three dimensional environment and the PCs may have to expend movement vertically as well as horizontally. Movement requires time. If their underwater foes fall back instead of advancing they use up a lot of their breath holding time just trying to get close enough for combat.
Twenty rounds under water does not seem so long if the characters have more to do in that time than just the clash of blows. Add a sunken ship or kelp forest for them to get tanged in and start a countdown timer. Underwater action when you can't breathe water can get scary in a hurry. be sure to start the "holding your breath" clock from the moment they stick their face under the surface.
This seems like a very good idea, sadly, i cannot put some homebrew rule in my game that changes something beneficing the players, because they enrage. I'm aware this is not the best quirk for a group but, as we say in spain, one has to know the sheeps he keeps.
This can work pretty well for a 20 round player submerged, and its completely true i should use the underwater environment to put pressure into the fight by adding dangers to the scenario. However, that's the amount of time that the wizard of my party can hold his breth, the rest of players, 4 in total, can hold it for a min of 30 rounds, even 40 rounds. Thats a lot time pursuing a foe, and even a lot of time to cut the weeds entangling them.
I dont want the underwater combat to kill them tho, i want it to make them feel like they are completely out of the comfort zone. Except for the triton in the party of course, that guy is obviously in his terrain. XD
I will try to apply these suggestions, they are all awsome.
Something else that might be possible to play with, is a series of encounters, underwater, where the Party is hop-scotching from one zone of safety to another. They start on the surface, and have to fight their way down to the underwater cave, which contains breathable air, from there, they have to swim through a series of fast current lava tubes until they get to the next pocket of air, etc.
This could give an interesting sensation of urgency ( they basically need to solve the next adventure barrier quickly, before they drown ), and a sensation that they are overextending themselves, as there is soon no way for them to quickly return to the surface and safety - they have to make their way back, through many of the same challenges ( or new challenges that have moved into the space of the old challenge they removed ).
You could - essentially - make a small classic Dungeon Crawl with a very interesting panicked urgency sub-aquatic atmosphere, and some serious time pressures ( solve the next barrier and get through it, before you drown! ).
This could be tied to any environment where exposure is quickly but not immediately lethal, as well - extreme heat, cave complexes filled with poison gas, vacuum of space ( if you have Spelljammers in your setting), etc. - although underwater ( and perhaps space ) would give you the interesting possibility of arranging your encounters three dimensionally.
I may have to file this one for my own use ;)
My DM Philosophy, as summed up by other people: https://drive.google.com/file/d/1rN5w4-azTq3Kbn0Yvk9nfqQhwQ1R5by1/view
Disclaimer: This signature is a badge of membership in the Forum Loudmouth Club. We are all friends. We are not attacking each other. We are engaging in spirited, friendly debate with one another. We may get snarky, but these are not attacks. Thank you for not reporting us.
Now this is very interesting. My group is heavy melee/martial advanced, a challenge underwater that involves crawling, stretching, even disabling a drowned trap that stills works can be very, VERY interesting. Also the dungeon may be some interesting and original thing, like beign trapped in the last floor of a big sinking boat or an underwater cave in the ruins of a lost elvish civilization....even maybe a jail in the lands of an evil triton lord.
Thanks for sharing your ideas, i think i can build something very nice from this
Thanks for all the tips and ideas! I am planning a new campaign and became fascinated with including an underwater section. :)
So tell them that if they want to play, they go by your rules! And that if they are that upset about the rules you put in place, they can leave but if they stay, your rule is law at the table, being the dungeon master.
From the Basic Rules/Player's Handbook section on Constitution Checks -
The DM might call for a Constitution check when you try to accomplish tasks like the following:
I'll be sending my children into shark infested waters in a week or two, so am trying to get my head around this topic...
There's a good argument to be had that just because a creature can hold its breath for 1+Con Mod minutes, doesn't mean they will (especially if they're being attacked). The 1+Con Mod rule for holding your breath is from the "Suffocating" section of the PHB - arguably this is a higher limit, not an automatically given allowance to 'spend' before you suffocate. I'd assume a check similar to spell concentration would be appropriate for holding breath while taking damage... although it is a bit strange that it isn't mentioned one way or the other in the Underwater Combat rules.
Remember that the players can be grappled, netted and the deep warlock can use grasp of hadar to keep pulling them down.
In fact, use a NPC guide that gets kidnapped this way to inspire terror.
Surprise round NPC receives eldritch bolt and gets pulled 20ft into the water. Next round is hit again and pulled 20ft under where it is netted and grappled by multiple Sahaugin with tridents.
by end of 1st active round npc is 40 ft away, surrounded by enemies and under at least 20 ft of water. 2nd round they leave with their food into the dark depths.
Vedexent put it really greatly. I ran an underwater arc at one point where I at least gave my party some diving gear that would break after three hits before somebody would have to start holding their breath. But there are certainly ways to make interesting engagements while helping the party consider the value of their oxygen and the limiters they might have by being underwater. The movement, the disadvantage, the limitations on capabilities, etc. are all far more difficult to work with in practice than you might initially expect- especially when the party likely is dealing with something that has an advantage underwater compared to them. I put the party against MTF's Leviathan at the bottom of the sea and BOY WAS THAT INTENSE! If you really want to push harder though, you can always as a DM choose to interpret rules how you would like. If you think that people should lose some oxygen when they get hit, tell the PC to roll a CON save as if they were concentrating and find a way to determine how many rounds of oxygen they lose by it. Underwater encounters are certainly a place for creating a great sense of isolation and urgency; if your party isn't packing easy outs like Water Breathing or Water Walk, make them feel the panic of being without a guarantee of survival!
I honestly wonder, who thinks drowning is fun? The threat of drowning is what makes things fun. With all the ways player characters can avoid it, you have to push very hard indeed to make them drown. Adventures under the water are a way to give people a new setting to interact with, it's not supposed to be something you kill their characters with directly, you do that with the monsters if you have to do it at all. I approach killing player characters as something I accept, because it happens sometimes, but I won't set out with the intention of doing it.
If you set out with the attitude that you're going to force your player's characters to fail by making a threat and then carrying it out, they are likely to find something better to do with their time sooner or later. Combat under water is fun. Drowning isn't.
<Insert clever signature here>
what if the party has people that can breath water?
Breathing is only one part of the equation. Most playable races don’t have a swim speed.
If they are going up against underwater fighters with blind fighting style, the enemy might churn up the water to blind their foes. This leaves the party without ranged attacks/spells and the enemy has advantage on attacks.
This actually works well on land too. Spellcaster casts “fog cloud” over area and a number of lvl 2 fighter/rogues with blind fighting engage in combat.
Undead that drag or push people into the water where a swarm of piranha wait.
Let the PC’s rescue the NPC only to have its water breathing dispelled, now they have to make the choice of rescuing the NPC or winning the fight.
Geann said it well. Drowning is not a fun mechanic to add to combat. It's a limitation on PCs that changes the equation on how to deal with a challenge. In short, you need to flee/avoid underwater combat unless you have the proper tools to handle it. For non-combat challenges, it can serve as a nice ticking clock.
My homebrew subclasses (full list here)
(Artificer) Swordmage | Glasswright | (Barbarian) Path of the Savage Embrace
(Bard) College of Dance | (Fighter) Warlord | Cannoneer
(Monk) Way of the Elements | (Ranger) Blade Dancer
(Rogue) DaggerMaster | Inquisitor | (Sorcerer) Riftwalker | Spellfist
(Warlock) The Swarm