I'm in the process of writing up a campaign for the summer that will be heavily focused on the coast/underwater. I have told my players, so obviously they are choosing (or at least they have spoken about choosing) races and subclasses that give them a swimming speed/allow them to breathe underwater. T
his has eliminated the problems of them staying below the waves for long periods of time and moving about. However, I'm currently stuck on creating combat encounters that are dynamic and have multiple parts.
In the current campaign we are playing in (I DM) one of the partys encounters was in a roofless temple, but the pillars were moving constantly due to being knocked over, or the players/npcs were able to climb up to attack each other from above. This meant the party had to think far more about positioning and what spells they could and couldn't use.
How am I meant to make encounters dynamic like this underwater? Much of the sea around the islands will be a giant coral reef, so players can duck and weave between holes/twists in the coral, but other than providing cover it doesn't feel like it does much.
Any help would be greatly appreciated.
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'The Cleverness of mushrooms always surprises me!' - Ivern Bramblefoot.
I’d think in terms of every encounter featuring everybody flying. Underwater, the fights will all play out in 3 dimensions with different heights (or I guess depths) for each creature involved. Have things coming from above and below, not just front and back. The duck behind the coral, it will help them find cover from some opponents, but maybe not ones who are high or low enough to see around it from other angles.
You could have a combat during a major tidal event, and everyone at the start of their turn needs to make a str save of be moved 20’ or something.
An underwater steam vent/ geyser that releases a plume of boiling water every so often, dex save or take fire damage.
And overall, really establish how far they’ll be able to reach. Personally, I like to go with occupying a 5’ cube as opposed to a square. Even if they are 6’ tall, and so would be in two cubes, for fight mechanics purposes, say they are only in one. A lot of the combat mechanics don’t work quite as well in 3 dimensions, so be prepared for some wonkiness, and probably warn your players of that.
From my knowledge of the ocean, the sea is relatively empty of obstruct aside natural corals reefs, and ships wrecks along the shore lines, so they would be greet spaces were the only variation in terrarian would be the elevation of the sea bed creating a rather uncover area for the players to deal with, unable to achieve any form of protection for a powerful enemy.
Corals and such also would end off at a certain height while they may have great protection for some space on the floor, anything above is open water, ship wrecks can offer to break that with the height and size of the ship, plus interior space. But how decomposed is this wreck, is it moslty buried and gone or wrecked recently.
Cliffs, and Underwater mountains. Outside the shoreline the terrarian varies greatly, with suddenly drop offs and mountains taller then Everest. Volcanic activity is also happens greatly along the ocean floors, so players would have interaction with lava, the quicky solidifies, but constantly bubbling up from the ground
The terrain of the ocean is 3D as others have said. It’s also at least as varied as the inland terrain types rangers work with. Before starting I would think some things thru - 1) what surface climate region(s) is this going to occur in? 2) how deep into the ocean are you planning to go? 3) what is the ultimate goal of the campaign? 4) what foes are you planning on using? 5) is everyone familiar with the rules for underwater combat and any additional house rules you are looking to use? Many fights will be in darkness or near darkness due to depth, sediment, obstructions, etc. are your players prepared for that?
one thing most folks don’t get is that water isn’t actually clear except in thin sections. If it were you could see the to the bottom of any ocean. Red light is absorbed in about 10 ft while blue and violet make it almost 1000 ft down before being absorbed leaving it black from there down. There can be huge (like miles across) blocks from landslides that might contain the ruins of ancient surface or subsea towns/cities/cvlizations. For terrains you have the open oceans, coral reefs, kelp beds, sea grass beds, cave and tunnels, canyon lands (with strong currents), sub ice, volcanic, vented, near shore, at least 4 different shoreline/tidal types and plenty more . Most life occurs n the upper nearshore (shelf) environments but some creatures (giant squid, kraken, aboleths) prefer the dark depths. As someone else said plan your fights more like fighter dogfights than like standard ground combats where there is a clear frontline and only 2 sides to control to avoid being flanked. If you want multi stage combats just have more “flights” of foes arrive from different directions.
Keep in mind that coral reefs are living environments. This means that creatures like eels, octopi, giant camouflaged crabs, etc.are likely to pop up to catch a quick bite of a PC, esp. if the PC is Small in size or resembles a fish. Also, spells like Sleep or Anti-Life Shell might not work as well in areas like reefs or at the ocean surface due to the large number of tiny, almost invisible creatures there.
The other thing to think about is that many spells would work differently. Gasses and terrain spells, esp.
As for other types of terrain, the skeleton of a giant or of something like a dragon turtle is not likely to decompose all that quickly in some areas of the ocean. I mean the bones and teeth will stick around for a while, possibly becoming a hotspot for scavengers and fish that rely on cover for safety.
Coral reefs and Kelp forests are alive, not static structures. Have anemones, and giant clams that are living traps, or octopi or eels that are hidden in the reef. Have corals that release clouds of spores or squid that release bursts of ink to disrupt line-of-sight. Other corals might grab onto creature, or sting them.
Corals can be destroyed as easily as buildings, some might be growing atop giant crabs that have buried themselves in the sand, that can get up and walk away if hit by AoEs.
Add in currents in the water that shift friend and foe alike on Initiative 20.
Also, spells like Sleep or Anti-Life Shell might not work as well in areas like reefs or at the ocean surface due to the large number of tiny, almost invisible creatures there.
I’d be very hesitant to go down that road. On land, there are any number of spiders, flies and mites around. It would get to be a real mess trying to account for every living creature in any given space.
The ocean has a large number of terrains, not just “coral reef”. You have rocky area with ridges and canyons like the Santa Cruz canyon, you have murky shallow swirling currents around rocky islands like San Francisco Bay, you have the kelp forests of the pacific coasts, you have the mangrove shallows of many tropical islands, the rocky shores and slopes of many lslands. You also have regions of sand bars and grass flats, you have shear cliffs, hot minereal springs, oil seep zones, 0 oxygen zones like the bottom of the Black Sea and sections of the floor of the Gulf of Mexico, Brine springs, wrecks ancient, recent and everything in between, dead whales and other large creatures, deep abyssal plans of soft mud, zones metallic ore nodules, slide blocks ranging from a few feet across to miles across and just about anything else you want to envision. Using earth as a guide you have pressures ranging from a few atmospheres to over 1100 atmospheres. Depending on how “gritty realism” you want to get you may have to deal with the “the bends” and or with nitrogen narcosis. I am sort of assuming that the campaign won’t occur in the deep sea or in the open ocean but rather in a section of the shelf and so no more than about 600’ deep and in the lighted zone which makes things somewhat easier.
Add in currents in the water that shift friend and foe alike on Initiative 20.
Coral reefs are really far more fragile than most materials we associate with buildings. They are easier to damage than stone or brick buildings, for instance. This is why fishing tends to be restricted around the more famous coral reefs: to protect them from the damage that is done by stuff like mass lobster traps, trawl nets, and anchors.
Currents of water shifting positions is a good idea. Me like.
Others have probably written this here, but I haven't read through them all yet, but making the terrain interesting could help differentiate the combat scenarios. Being the water allows the battles to act in three dimensions (this was mentioned by a bunch of people) which allows for complex flanking formation from more than side to side of back to front, offering an up down and more diagonals. As for keeping the terrain interesting, perhaps you could try throwing in whirlpools, ships floating on the sea for the party to tackle with from below, or to assist. As others have mentioned, coral reefs are also great underwater structures, and in addition to them you could throw in lost cities overtaken by the sea in the ancient past, sea elf and sahuagin nations that have different architectural practices and so on. Finally, depending on the levels for the campaign, Storm Giants have been known to live at the bottom of the ocean, and as such may be a high level encounter to provide for the party.
Aside from all of this, utilizing the edge of the ocean, with beaches, the surface, or the seafloor and trenches, may be some of the best natural devices for interesting sea adventures.
I've also been trying to build out an interesting under sea adventure, and have used the above to varying degrees.
Also, spells like Sleep or Anti-Life Shell might not work as well in areas like reefs or at the ocean surface due to the large number of tiny, almost invisible creatures there.
I’d be very hesitant to go down that road. On land, there are any number of spiders, flies and mites around. It would get to be a real mess trying to account for every living creature in any given space.
Or we could think of it as in incentive to diversify one's spell selection.
Counting creatures that don't have stat blocks against the effects of Sleep or other spells falls more under "the GM is being a jerk" than "incentive to diversify spell selection."
Besides, most animals would hide or flee as soon as combat broke out so they either wouldn't be in the area of effect or would have total cover from it.
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Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
Counting creatures that don't have stat blocks against the effects of Sleep or other spells falls more under "the GM is being a jerk" than "incentive to diversify spell selection."
Besides, most animals would hide or flee as soon as combat broke out so they either wouldn't be in the area of effect or would have total cover from it.
No, it's about how game-fied D&D has become. As in video gamey. This is precisely the problem with trying to make a tabletop experience imitate an ARPG or video game. People get used to thinking in terms of video game rules instead of what actually MAKES SENSE in terms of the game world itself. It's no wonder that OneD&D developers are pushing to "streamline" the game and make it easier to program: their gamble for the success of a VTT platform benefits from this type of narrow thinking where everything exists exclusively to serve as plot or combat points for the players.
Also:
Player: I want to cast Sleep on the Sea Troll.
DM: Are you sure? Since you're swimming on the ocean surface, there's billions of plankton all around your target.
Player: I guess not. Okay, I'll use Silent Image instead. The image is of a block of ice separating the Sea Troll from me and Thanagrid.
IOW, it's not "being mean to the Player" if the DM just tells them of likely unforeseen consequences. Communication, as usual, resolves a lot of potential problems.
Counting creatures that don't have stat blocks against the effects of Sleep or other spells falls more under "the GM is being a jerk" than "incentive to diversify spell selection."
Besides, most animals would hide or flee as soon as combat broke out so they either wouldn't be in the area of effect or would have total cover from it.
No, it's about how game-fied D&D has become.
Playing that card negates any validity your argument might have had. I've been playing this game since 2nd Edition and there is no point where it would have been acceptable to claim that a bunch of statless, 0 HP critters just absorb a spell like and negate its effects on the actual target. In the entire time I've been playing, there has never been a point where people haven't been trying to claim that the game is too video gamey now, and there has never been a point where that argument actually contributed anything to a discussion.
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Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
LIke I said in my edited post, if you don't want to come off as "mean DM" just communicate with the Player that their spell might not function as they have come to expect from their video game-ified experience of D&D before the spend their spell slot. That's all it takes.
How is that absurd? Just because marine biomass of phytoplankton is largely invisible to the naked human eye doesn't make it insignificant. Almost the entire ocean ecosystem is sustained on phytoplankton . Krill, the zooplankton consumed by the largest animal to have ever existed (the blue whale) feeds on phytoplankton. Most fish eat the zooplankton that eats phytoplankton or other zooplankton that eats phytoplankton. The food pyramid of the ocean is dependent on invisible creatures.
Would you say that a swarm of rats or swarm of insects who are large enough hit point-wise to feed a wolf for a week to be insignificant? Just because marine systems are little understood by the game devs and largely neglected by the Monster Manual does not mean that somehow swarms of nano-creatures stop existing for the convenience of the PCs. That's like saying that NPCs only exist while the PCs are around to talk to them/steal from them/kill them. That's what's absurd: the use of video game logic in an open world tabletop RPG.
No one is disputing the amount of tiny critters in the ocean. The point is, there are tiny critters everywhere. Spiders, dust mites, sparrows, mice. Once you start saying all these background animals are effected, sleep only works in sterile, clean room laboratory conditions because you’ve managed to knock out 17hp worth of lice clinging to the wolf before the wolf itself is effected.
No one is disputing the amount of tiny critters in the ocean. The point is, there are tiny critters everywhere. Spiders, dust mites, sparrows, mice. Once you start saying all these background animals are effected, sleep only works in sterile, clean room laboratory conditions because you’ve managed to knock out 17hp worth of lice clinging to the wolf before the wolf itself is effected.
There is a difference, though. Half the critters you mention have clear reasons not to be near Small or Medium sized creatures because they are afraid of being preyed upon. Unless someone is encouraging them by feeding them, mice and sparrows are not going to casually hang out within a few feet of most NPCs. If spiders or mites are in large enough numbers, then, yes, they both would be visible and would affect a spell like Sleep. But a handful of regular sized spiders or a dozen mites would hardly constitute more than 2 hit points.
What is distinctive about ocean ecosystems is that phytoplankton have taken the place of plants in the land-based environments that means they make up a huge portion of the marine biomass that uses photosynthesis to generate nutrients and sugars to survive/reproduce. And most of the creatures that eat phytoplankton are also mostly quite small. While many are microscopic, many others would be visible when gathered in large enough numbers. Krill is a good (if particularly large) example. Individual krill are about the length of a matchstick. The larval versions of jellyfish, sea anemones, sea urchins, coral, and many other very visible sea creatures also count as zooplankton (zoo = animal).
Coral reefs are made up of thousands and thousands of mature zooplankton. See 4:59 of this YouTube video: Keystone Species of the Sea.
The relationship between whales and plankton is talked about starting at 9:38 of this YouTube video: Keystone Species of the Sea.
When zooplankton band together, they can produce very visible results, like the Portuguese Man of War, often mistaken for jellyfish. See this video about Siphonophores.
Also, scientists estimate that phytoplankton produce about 50% of the world's oxygen. This is the very definition of having a significant impact. Reference this video by the Hakai Institute about the little known role of plankton to not just ocean ecosystems, but to GLOBAL ecosystems.
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I'm in the process of writing up a campaign for the summer that will be heavily focused on the coast/underwater. I have told my players, so obviously they are choosing (or at least they have spoken about choosing) races and subclasses that give them a swimming speed/allow them to breathe underwater. T
his has eliminated the problems of them staying below the waves for long periods of time and moving about. However, I'm currently stuck on creating combat encounters that are dynamic and have multiple parts.
In the current campaign we are playing in (I DM) one of the partys encounters was in a roofless temple, but the pillars were moving constantly due to being knocked over, or the players/npcs were able to climb up to attack each other from above. This meant the party had to think far more about positioning and what spells they could and couldn't use.
How am I meant to make encounters dynamic like this underwater? Much of the sea around the islands will be a giant coral reef, so players can duck and weave between holes/twists in the coral, but other than providing cover it doesn't feel like it does much.
Any help would be greatly appreciated.
'The Cleverness of mushrooms always surprises me!' - Ivern Bramblefoot.
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I’d think in terms of every encounter featuring everybody flying. Underwater, the fights will all play out in 3 dimensions with different heights (or I guess depths) for each creature involved. Have things coming from above and below, not just front and back. The duck behind the coral, it will help them find cover from some opponents, but maybe not ones who are high or low enough to see around it from other angles.
You could have a combat during a major tidal event, and everyone at the start of their turn needs to make a str save of be moved 20’ or something.
An underwater steam vent/ geyser that releases a plume of boiling water every so often, dex save or take fire damage.
And overall, really establish how far they’ll be able to reach. Personally, I like to go with occupying a 5’ cube as opposed to a square. Even if they are 6’ tall, and so would be in two cubes, for fight mechanics purposes, say they are only in one. A lot of the combat mechanics don’t work quite as well in 3 dimensions, so be prepared for some wonkiness, and probably warn your players of that.
From my knowledge of the ocean, the sea is relatively empty of obstruct aside natural corals reefs, and ships wrecks along the shore lines, so they would be greet spaces were the only variation in terrarian would be the elevation of the sea bed creating a rather uncover area for the players to deal with, unable to achieve any form of protection for a powerful enemy.
Corals and such also would end off at a certain height while they may have great protection for some space on the floor, anything above is open water, ship wrecks can offer to break that with the height and size of the ship, plus interior space. But how decomposed is this wreck, is it moslty buried and gone or wrecked recently.
Cliffs, and Underwater mountains. Outside the shoreline the terrarian varies greatly, with suddenly drop offs and mountains taller then Everest. Volcanic activity is also happens greatly along the ocean floors, so players would have interaction with lava, the quicky solidifies, but constantly bubbling up from the ground
The terrain of the ocean is 3D as others have said. It’s also at least as varied as the inland terrain types rangers work with. Before starting I would think some things thru -
1) what surface climate region(s) is this going to occur in?
2) how deep into the ocean are you planning to go?
3) what is the ultimate goal of the campaign?
4) what foes are you planning on using?
5) is everyone familiar with the rules for underwater combat and any additional house rules you are looking to use? Many fights will be in darkness or near darkness due to depth, sediment, obstructions, etc. are your players prepared for that?
one thing most folks don’t get is that water isn’t actually clear except in thin sections. If it were you could see the to the bottom of any ocean. Red light is absorbed in about 10 ft while blue and violet make it almost 1000 ft down before being absorbed leaving it black from there down. There can be huge (like miles across) blocks from landslides that might contain the ruins of ancient surface or subsea towns/cities/cvlizations. For terrains you have the open oceans, coral reefs, kelp beds, sea grass beds, cave and tunnels, canyon lands (with strong currents), sub ice, volcanic, vented, near shore, at least 4 different shoreline/tidal types and plenty more . Most life occurs n the upper nearshore (shelf) environments but some creatures (giant squid, kraken, aboleths) prefer the dark depths. As someone else said plan your fights more like fighter dogfights than like standard ground combats where there is a clear frontline and only 2 sides to control to avoid being flanked. If you want multi stage combats just have more “flights” of foes arrive from different directions.
Wisea$$ DM and Player since 1979.
Keep in mind that coral reefs are living environments. This means that creatures like eels, octopi, giant camouflaged crabs, etc.are likely to pop up to catch a quick bite of a PC, esp. if the PC is Small in size or resembles a fish. Also, spells like Sleep or Anti-Life Shell might not work as well in areas like reefs or at the ocean surface due to the large number of tiny, almost invisible creatures there.
The other thing to think about is that many spells would work differently. Gasses and terrain spells, esp.
As for other types of terrain, the skeleton of a giant or of something like a dragon turtle is not likely to decompose all that quickly in some areas of the ocean. I mean the bones and teeth will stick around for a while, possibly becoming a hotspot for scavengers and fish that rely on cover for safety.
Coral reefs and Kelp forests are alive, not static structures. Have anemones, and giant clams that are living traps, or octopi or eels that are hidden in the reef. Have corals that release clouds of spores or squid that release bursts of ink to disrupt line-of-sight. Other corals might grab onto creature, or sting them.
Corals can be destroyed as easily as buildings, some might be growing atop giant crabs that have buried themselves in the sand, that can get up and walk away if hit by AoEs.
Add in currents in the water that shift friend and foe alike on Initiative 20.
I’d be very hesitant to go down that road. On land, there are any number of spiders, flies and mites around. It would get to be a real mess trying to account for every living creature in any given space.
so one thing coral is very sharp and towards the the top of the water the ocean can slam you in so that could be a mechanic
The ocean has a large number of terrains, not just “coral reef”. You have rocky area with ridges and canyons like the Santa Cruz canyon, you have murky shallow swirling currents around rocky islands like San Francisco Bay, you have the kelp forests of the pacific coasts, you have the mangrove shallows of many tropical islands, the rocky shores and slopes of many lslands. You also have regions of sand bars and grass flats, you have shear cliffs, hot minereal springs, oil seep zones, 0 oxygen zones like the bottom of the Black Sea and sections of the floor of the Gulf of Mexico, Brine springs, wrecks ancient, recent and everything in between, dead whales and other large creatures, deep abyssal plans of soft mud, zones metallic ore nodules, slide blocks ranging from a few feet across to miles across and just about anything else you want to envision. Using earth as a guide you have pressures ranging from a few atmospheres to over 1100 atmospheres. Depending on how “gritty realism” you want to get you may have to deal with the “the bends” and or with nitrogen narcosis. I am sort of assuming that the campaign won’t occur in the deep sea or in the open ocean but rather in a section of the shelf and so no more than about 600’ deep and in the lighted zone which makes things somewhat easier.
Wisea$$ DM and Player since 1979.
Coral reefs are really far more fragile than most materials we associate with buildings. They are easier to damage than stone or brick buildings, for instance. This is why fishing tends to be restricted around the more famous coral reefs: to protect them from the damage that is done by stuff like mass lobster traps, trawl nets, and anchors.
Currents of water shifting positions is a good idea. Me like.
Others have probably written this here, but I haven't read through them all yet, but making the terrain interesting could help differentiate the combat scenarios. Being the water allows the battles to act in three dimensions (this was mentioned by a bunch of people) which allows for complex flanking formation from more than side to side of back to front, offering an up down and more diagonals. As for keeping the terrain interesting, perhaps you could try throwing in whirlpools, ships floating on the sea for the party to tackle with from below, or to assist. As others have mentioned, coral reefs are also great underwater structures, and in addition to them you could throw in lost cities overtaken by the sea in the ancient past, sea elf and sahuagin nations that have different architectural practices and so on. Finally, depending on the levels for the campaign, Storm Giants have been known to live at the bottom of the ocean, and as such may be a high level encounter to provide for the party.
Aside from all of this, utilizing the edge of the ocean, with beaches, the surface, or the seafloor and trenches, may be some of the best natural devices for interesting sea adventures.
I've also been trying to build out an interesting under sea adventure, and have used the above to varying degrees.
Or we could think of it as in incentive to diversify one's spell selection.
Counting creatures that don't have stat blocks against the effects of Sleep or other spells falls more under "the GM is being a jerk" than "incentive to diversify spell selection."
Besides, most animals would hide or flee as soon as combat broke out so they either wouldn't be in the area of effect or would have total cover from it.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
No, it's about how game-fied D&D has become. As in video gamey. This is precisely the problem with trying to make a tabletop experience imitate an ARPG or video game. People get used to thinking in terms of video game rules instead of what actually MAKES SENSE in terms of the game world itself. It's no wonder that OneD&D developers are pushing to "streamline" the game and make it easier to program: their gamble for the success of a VTT platform benefits from this type of narrow thinking where everything exists exclusively to serve as plot or combat points for the players.
Also:
Player: I want to cast Sleep on the Sea Troll.
DM: Are you sure? Since you're swimming on the ocean surface, there's billions of plankton all around your target.
Player: I guess not. Okay, I'll use Silent Image instead. The image is of a block of ice separating the Sea Troll from me and Thanagrid.
IOW, it's not "being mean to the Player" if the DM just tells them of likely unforeseen consequences. Communication, as usual, resolves a lot of potential problems.
Playing that card negates any validity your argument might have had. I've been playing this game since 2nd Edition and there is no point where it would have been acceptable to claim that a bunch of statless, 0 HP critters just absorb a spell like and negate its effects on the actual target. In the entire time I've been playing, there has never been a point where people haven't been trying to claim that the game is too video gamey now, and there has never been a point where that argument actually contributed anything to a discussion.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
LIke I said in my edited post, if you don't want to come off as "mean DM" just communicate with the Player that their spell might not function as they have come to expect from their video game-ified experience of D&D before the spend their spell slot. That's all it takes.
That's not being creative, that's being arbitrary and absurd. It doesn't matter whether you tell the player beforehand or not.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
How is that absurd? Just because marine biomass of phytoplankton is largely invisible to the naked human eye doesn't make it insignificant. Almost the entire ocean ecosystem is sustained on phytoplankton . Krill, the zooplankton consumed by the largest animal to have ever existed (the blue whale) feeds on phytoplankton. Most fish eat the zooplankton that eats phytoplankton or other zooplankton that eats phytoplankton. The food pyramid of the ocean is dependent on invisible creatures.
Would you say that a swarm of rats or swarm of insects who are large enough hit point-wise to feed a wolf for a week to be insignificant? Just because marine systems are little understood by the game devs and largely neglected by the Monster Manual does not mean that somehow swarms of nano-creatures stop existing for the convenience of the PCs. That's like saying that NPCs only exist while the PCs are around to talk to them/steal from them/kill them. That's what's absurd: the use of video game logic in an open world tabletop RPG.
No one is disputing the amount of tiny critters in the ocean. The point is, there are tiny critters everywhere. Spiders, dust mites, sparrows, mice. Once you start saying all these background animals are effected, sleep only works in sterile, clean room laboratory conditions because you’ve managed to knock out 17hp worth of lice clinging to the wolf before the wolf itself is effected.
There is a difference, though. Half the critters you mention have clear reasons not to be near Small or Medium sized creatures because they are afraid of being preyed upon. Unless someone is encouraging them by feeding them, mice and sparrows are not going to casually hang out within a few feet of most NPCs. If spiders or mites are in large enough numbers, then, yes, they both would be visible and would affect a spell like Sleep. But a handful of regular sized spiders or a dozen mites would hardly constitute more than 2 hit points.
What is distinctive about ocean ecosystems is that phytoplankton have taken the place of plants in the land-based environments that means they make up a huge portion of the marine biomass that uses photosynthesis to generate nutrients and sugars to survive/reproduce. And most of the creatures that eat phytoplankton are also mostly quite small. While many are microscopic, many others would be visible when gathered in large enough numbers. Krill is a good (if particularly large) example. Individual krill are about the length of a matchstick. The larval versions of jellyfish, sea anemones, sea urchins, coral, and many other very visible sea creatures also count as zooplankton (zoo = animal).
Coral reefs are made up of thousands and thousands of mature zooplankton. See 4:59 of this YouTube video: Keystone Species of the Sea.
The relationship between whales and plankton is talked about starting at 9:38 of this YouTube video: Keystone Species of the Sea.
When zooplankton band together, they can produce very visible results, like the Portuguese Man of War, often mistaken for jellyfish. See this video about Siphonophores.
Also, scientists estimate that phytoplankton produce about 50% of the world's oxygen. This is the very definition of having a significant impact. Reference this video by the Hakai Institute about the little known role of plankton to not just ocean ecosystems, but to GLOBAL ecosystems.