Under the Sea

"Under the sea! Under the sea! There'd be no accusations, just friendly crustaceans, Under the seaaaaaaa!"

- Homer, The Simpsons, parodying The Little Mermaid

"Wait, if we're underwater, how did we start a fire?"

- Patrick Star, SpongeBob SquarePants

Aquatic levels have been a staple of Nintendo games since Super Mario Bros., but can be seen in other games as well. Typical underwater enemies are sharks, octopuses, electric eels and jellyfish. Expect some Underwater Ruins every now and again for decoration.

Human heroes, such as Mario, often swim fully clothed, without being weighed down or otherwise impeded.

Of course, there are three problems with the underwater level: one, swimming, two, slower movement, and three, drowning. (Or not.) Down the Drain levels are especially prone to special water-based hazards and hindrances, as detailed on that page.

Compare Shark Tunnel. Many of those might enter That One Level territory.

Named for the Oscar-winning song from The Little Mermaid.

Action Adventure

 * Water levels abound in almost every The Legend of Zelda game, though the biggest was probably The Wind Waker, as the entire game takes place on a giant ocean. At a point in the game, Link can also meet what's beneath the water.
 * Ocarina of Time's Water Temple, however, was so bad that the Master Quest rerelease (which upped the difficulty in all levels) is said to be easier than the original. Which means they replaced one of the rooms with one that had half a dozen Stalfos in it. Problems include slow mobility through the submerged hallways, and backtracking between water level switch areas numerous times, which involved many treks through said hallways. Time after time you had to go in circles, Hookshotting every which way to collect keys, adjust water levels, and avoid enemies with indestructible shells. Then the battle with Dark Link will take a large amount of HP out of you, though it can be replaced by MP loss if you have Din's Fire, and both could be lowered by doing the trading sidequest for the Biggoron's Sword (or the Megaton Hammer, which most players will have by that point in the game). And then when you get the Longshot, you have to go down this river where there are several whirlpools that act like bottomless pits, forcing you to start over. And then you have to get the Big Key to fight Morpha, who is relatively easy, but has a chance to do 5 Hearts of damage by simply using his standard attack, if you're unlucky.
 * The Nintendo 3DS remake had the Water Temple as the only dungeon with substantial changes beyond the obvious cosmetic changes, such as color-coding the paths that you need to follow to change the water level, and making the Iron Boots a button-based item, eliminating the menu-switching problem.
 * Primal has a full-blown underwater level. Jen is given the Undine form to work and fight underwater while swimming. In an inversion of Super Drowning Skills, Jen's water form loses health when out of water, and cannot take so much as a step on dry land. Scree, being made of stone, just sinks and walks.
 * The Dragon Palace in Okami is completely underwater, but the controls are identical to playing above land - jumping, gravity, et al. That includes small ponds that you can drown in. Underwater. (You can also shake off this underwater water)
 * Blaster Master's Stage 5, in addition to having a That One Boss, is almost entirely underwater, so your Rover moves and jumps really slowly, and you don't get the swimming ability for it until after you beat the boss, so to reach certain parts you are forced to swim in just your space suit, making you a sitting duck for the otherwise weak Mooks.
 * DeepDive from An Untitled Story. A vertical shaft of water. Your oxygen is limited, but there are several air sources and Oxygen Meter expansions to survive.
 * The Metroidvania Castlevania games often have a water level. Circle of the Moon's is mostly only shallow puddles of water, and Portrait of Ruin has just a single boss, but the rest feature full underwater levels where you're slower and you can jump infinitely in the water. Usually an upgrade is needed to get this far. Order of Ecclesia has two such levels.
 * Alundra has the optional dungeon Fairy Pond which takes place mostly underwater and houses one of the best swords in the game in it.
 * Half of the NES game Jaws is fighting sealife underwater. Other half has you sailing your boat between two ports purchasing power ups.
 * Jabless Adventure has a few underwater areas, featuring fish enemies, a mermaid, and a SCUBA-diving bear.
 * The Teenage Mutant Ninja Turtles game for NES had the dam stage where you had to disarm eight bombs underwater.

Action Game

 * Taito's Jungle King and its lawyer-friendly variation, Jungle Hunt, have one level where the hero attacks crocodiles while swimming.
 * The submarine levels in various Metal Slug games.
 * Stage 5 of Metal Slug 4 takes place on a military cruiser, with pirates put in as the Middle-Eastern sword throwers from Metal Slug 2 with recolored sprites. Of course, the stage itself can be quite difficult until you manage to work your way around the many projectiles. Click to see the level.
 * In Spider-Man and the X-Men: Arcade's Revenge, Storm's levels take place underwater, presumably so the developers wouldn't have to deal with the fact that Storm can fly. These levels are actually the least Nintendo Hard in the game.
 * Tomb Raider 2 has large part of the game set in a sunken ship, and Tomb Raider: Chronicles has a section of the game set in a submarine, and the sea around it.
 * Tomb Raider underworld fits this trope more than the past titles as some of the stages take places in the Mediterranean sea and later the Arctic sea with underwater temples and so it has more emphasis in underwater gameplay.
 * The filtration plant in Shatterhand.
 * Captain America and The Avengers has Scene 3: "Challenge from the Bottom of the Sea," which is only part underwater.

Fighting Game

 * The final fight in Virtua Fighter 2 was underwater, slowing your movement.

First-Person Shooter

 * The "Pelagic II" and "Deep Sea" missions in Perfect Dark.

MMORPGs

 * The climax of zOMG's first chapter take place almost entirely under the ocean. Luckily, you've just been granted the power to breathe underwater.
 * There's also Aqua Road of Maple Story.
 * Kingdom of Loathing has underwater zones for leveling up post-ascension, but they're the least attractive option due to poor returns. Adventuring there costs 2 adventures rather than the standard 1, requires equipment that eliminates item drops and removes or reduces familiar bonuses, and the areas do not drop money.
 * Vashj'ir in World of Warcraft.

Platform Game

 * In Banjo-Tooie, Jolly Roger's Lagoon starts out as a Gang Plank Galleon level, but then they remove your Oxygen Meter...
 * Most worlds in Donkey Kong Country, and Donkey Kong 64, include one aquatic level. In these levels, the Kongs can ride or must transform into Enguarde the swordfish to repel enemies. Donkey Kong Jungle Beat was the first game Donkey can fight off enemies underwater without any animal buddies. Given the Kong's species, none of them should be able to swim in the first place.
 * This setting is a staple of the Mario series:
 * Super Mario Bros. was, of course, the first to feature a swimming Mario.
 * Super Mario Land 2: 6 Golden Coins has two such areas: the Hippo Zone, which can lead to the Space Zone, and the Turtle Zone.
 * Super Mario Bros 3 has a frog suit that allows Mario or Luigi to swim faster. Makes World 3, a water-based level, much easier.
 * In Super Mario World, Mario or Luigi can go underwater, also while riding Yoshi.
 * Super Mario 64 was the first to include an Oxygen Meter, and water with magical healing properties.
 * Super Mario Sunshine had elements of this in nearly every single level, but Noki Bay had Mario go underwater with a diving helmet in various underwater ruins type areas.
 * Super Mario Galaxy has a whole seven worlds/areas with aspects of this setting, with the usual water based enemies that go with it.
 * In Mario & Luigi: Superstar Saga, the brothers can walk on the ocean floor and even use their hand powers (fire and electricity) underwater.
 * Most Mega Man games have one of these; while the blue bomber didn't swim except in Mega Man 8 (almost-sentient robots must be quite heavy, he did have a Rush upgrade for it in some games though), being underwater made you jump higher, not that it was of any help.
 * Mega Man Star Force 2 had Mess Cove.
 * Undertow in Ristar.
 * Notably, while many Sonic the Hedgehog games contain water, Sonic can't swim, and instead runs along the floor, breathing air from bubbles made by underwater plants.
 * Though if he gets up enough speed in Hydrocity, he can skip the whole swimming thing and just run across the water.
 * As opposed to Sonic's Super Drowning Skills, Tails, Knuckles and Rouge can swim, and Knux can even get an artificial breather in Sonic Adventure 2 to dive as long as he wants (and even walk on the floor).
 * In Sonic Advance 3, Knuckles can swim on top of the water and optionally dive, while anyone with Cream as their partner has an eternal air bubble on their head when they go into water. Of course the worst part of the water levels were nightmares thanks to the drowning music.
 * Some actual examples include the Underwater Ruins levels such as Hydrocity, but Coral Cave in Sonic Rush Series Adventure mixes it up with Underground Level and Minecart Madness.
 * Aquarium Park Zone from Sonic Colors, the first underwater 3D Sonic level since Sonic Adventure 2.
 * The Labyrinth of the Sea from Sonic Labyrinth.
 * Labryinth Zone in the original Sonic the Hedgehog
 * Aquatic Ruin from Sonic the Hedgehog 2 and Hydrocity from Sonic 3 and Knuckles, both of which are also Underwater Ruins.
 * Tidal Plant in Sonic Triple Trouble which borders on being Down the Drain as you spend almost the entire second act underwater.
 * Metroid:
 * Maridia in Super Metroid, and the sunken frigate in Metroid Prime.
 * Sector 4 (AQA) in Metroid Fusion, and the Torvus Undertemple in Metroid Prime 2: Echoes.
 * Wario Land has had a few of these settings. The first had water levels in Sherbet Land and the SS Tea Cup areas, the second had the Ruins Under the Sea and the fourth game had Mystic Lake (not quite the sea, but it had the marine wildlife and features you'd see in an ocean).
 * Wario Land Shake It has various underwater submarine shooter type levels (Wavy Waters and Creep Blue Sea being examples)
 * Quaria in Bug!!. Filled with many fish enemies, fire-spitting moray eels (yes, underwater), crabs, starfish and clams. Oh, and the obligatory giant octopus/squid, which served as the boss.
 * Athena has the World of Sea, where Athena can use a necklace to transform herself into a mermaid.
 * In Commander Keen Episode 4: Secret of the Oracle, the Well of Wishes was the obligatory underwater level where you can't jump or shoot, and swimming controls are rather awkward. Its waters are home to numerous mines and the one and only Dopefish. Water in other places simply kills you at a touch.
 * In Spyro the Dragon, Spyro had Super Drowning Skills, so the first game didn't use water. However, subsequent games gave him the ability to swim and sent him to levels that were mainly water, which were usually very annoying.
 * Rayman Origins had several underwater levels, but they averted the frustrating part with fantastic swimming controls, and you could actually be just as agile as you are on land.
 * Little Nemo the Dream Master has the fourth level, Night Sea.

Puzzle Game

 * Repton Around the World includes the Oceans scenario, considered to be the game's most difficult.

Role-Playing Game

 * Atlantica in Kingdom Hearts, which is a world based on The Little Mermaid, the Trope Namer. Unfortunately, many players didn't like the level thanks to its unintuitive control scheme, so the developers "fixed" it for the sequel by replacing it with a (thankfully optional) rhythm game.
 * The Atlantis mission in Marvel Ultimate Alliance The difficulty of operating underwater is technobabbled away by the presence of special nanites that allow the heroes to survive and fight even while underwater. Considering characters like Storm, Silver Surfer and Ghost Rider can already do fine underwater, they could have just let everyone else sit out.
 * The final quest on Manaan in Knights of the Old Republic requires you to walk at the bottom of the ocean in a protective suit that slows you down to turtle-speed, while avoiding the dangerous wildlife. For many players, it counts as That One Level.
 * Wizardry 8 has the Bayjin Shallows and Mount Gigas Water Caves. Despite preventing fire magic from working, the water areas are not the resident Scrappy Level. That title is reserved for Bayjin, the Palmtree Panic level you're trekking underwater to get to.
 * In Pokémon Ruby and Sapphire, the Dive HM makes it possible to go underwater in certain regions of Hoenn. In fact, this is the only way to reach certain areas, like the entrance to Sootopolis City. Then Dive returns in Pokémon Black and White, though it doesn't see nearly as much use.
 * Bottomsup Bay in Fossil Fighters. The player walks fully-clothed along the bottom of the ocean, looking for fossils. There are also sharks when the area is first unlocked.
 * Mother 3 has the Sea Floor Dungeon between Tazmily Village and Tanetane Island. Lucas and company can't run underwater, and can only replenish their oxygen meter by kissing robotic mermen who breathe the air into them.
 * The Shoals in Might and Magic VII, entered late in the game. You can't cast magic, you have to wear wetsuits (which prohibits you from wearing or using any other kind of equipment, except for Blasters) and the mechanism for getting you there means you can't prepare yourself for it by casting buffs before you enter. On the plus side, there's no problem with swimming (it works like flying, except you sink - slowly - if you stop), and the sharks infesting the area are weak enough that by the time you reach the Shoals, the only reason they are something of a threat is the aforementioned limitations on magic and equipment.

Shoot'Em Up

 * Star Fox 64 has Aquas, which switches out the space fighter for a submarine. The original Star Fox doesn't have an underwater level per se, but Sector Y takes Space Is an Ocean to ridiculous levels.

Sports Game

 * One field in Backyard Baseball is the Aquadome, which is completely underwater. Yet the characters can still breathe...

Literature

 * Frank Herbert's Under Pressure.
 * Frederik Pohl and Jack Williamson's Undersea Trilogy.
 * Jules Verne's Twenty Thousand Leagues Under the Sea, making this Older Than Radio.
 * The latter portions of JRR Tolkien's Roverandom take place here.

Theme Parks

 * Disney Theme Parks: The Sub rides, later Finding Nemo attraction, and the newly opened The Little Mermaid dark ride.

Video Games

 * The Hunt for Red October games for the NES and SNES, where you play as the Red October.
 * Aquaria is a Metroidvania indie-darling game that takes place entirely in an undersea world.
 * BioShock (series) takes place in a city at the bottom of the ocean. Amusingly, there's absolutely no swimming sections. The only time you actually travel in the ocean are the level-transitioning bathyspheres, which are automatic.
 * The sequel adds underwater sections, but you can't use any weapons, you have infinite air, and no enemies appear while outside of the Underwater City.
 * The entirety of the Ecco the Dolphin series is spent underwater. And it's completely justified, because you are literally playing as a dolphin. Strangely enough, it works, despite the occasional Scrappy Level and Those Several Bosses... Did I mention Space Is an Ocean?
 * Endless Ocean
 * Any SpongeBob SquarePants game, obviously. However, it gets weird in the Nicktoons Unite! series where they have to explain how humans can breathe underwater. The first game technobabbles away with Jimmy Neutron's air gums and Attack of the Toybots skirts the issue by having Bikini Bottom as a tutorial level played only as SpongeBob (though Dummied Out voice clips suggest that it would have been a regular level playable with anyone) but Globs of Doom is an offender in choosing to give no explanation.
 * Dive II Hunt, a game which involves a known character from the Ivalice Alliance, Sorbet, scuba diving under the sea and obviously, to be the best hunter.
 * The Legendary Starfy
 * The classic submarine shooting game, In the Hunt. Well, except the final level, which was an enemy base.
 * The Little Mermaid Licensed Game sets all its levels underwater. Ariel doesn't turn back into a human and walk on dry land until the ending, which ironically is the only part of the game playing "Under the Sea" aside from the title screen.
 * The whole of Jaws Unleashed, excluding the trip to sea park and the tunnels of Environplus' undersea facility.
 * X-COM: Terror from the Deep
 * Swim Ikachan takes place in a sea cave cut off from undersea civilization by earthquakes and run by the tyrannical Ironhead.

Western Animation

 * The Little Mermaid 3: Daughters of Triton took place entirely underwater.
 * Finding Nemo was almost entirely set underwater, the characters being fish and all.
 * Re Boot has a game that was entirely underwater. It was used to introduce Andraia.
 * Jabberjaw tookplace in an era where mankind had developed civilizations underwater.