Your basic Mook, now for aquatic environments. They generally come in two types: an enemy specifically designed for aquatic levels, typically based on marine animals, and regular land-dwelling mooks repurposed for a new environment by being equipped with scuba gear (masks and air tanks), becoming more fish-like, or the like. This second option most commonly occurs with the Mascot Mook. Regardless of type, you'll definitely encounter them Under the Sea, Down the Drain and even in the Bubblegloop Swamp.
If it is an underwater level, then the hero is also given swimming skills so they can face these mooks as if they were on land; the difficulty in this case is that the jumping on head option is usually taken away, making it impossible to avoid Collision Damage (and forcing you to just avoid these guys) unless some other weapon is given. The other option (when on land) is that these guys jump in and out of water where they are unapproachable because of the hero's Super Drowning Skills, so you have to hit them while they are out of it (if they can be damaged, that is). Some games (like Super Mario Bros.) may have the same enemy show up in both roles, others will have different ones for the two environments.
- Ecco the Dolphin is packed with hordes of these, given that most of the series takes place under the ocean.
- The Legend of Zelda: In 2D games, this role is filled by the River Zoras, fish people who sit in the water, and thus well out of Link's reach unless he's obtained whatever item lets him swim in a given game, and spit fireballs at him. In 3D games, their place is largely taken by Octoroks, who behave in the same manner. The Legend of Zelda: Twilight Princess features neither enemy, but has Toadpolis who behave in the exact same manner.
- Contra: The arcade version has two of these that jump out of the water at the beginning of the game, and nowhere else.
- Justice League Heroes: The Flash has an aquatic Mecha-Mook introduced in the Sunken City level. They start with a water barrier that protects them, and after you destroy it they'll sometimes return to the water to replace it.
- 8Bit Killer: In stage 3-2, you encounter gun-toting enemies who use their diving suits to submerge in water to catch you by surprise.
- Deus Ex has a few Diver enemies on the way back from the underwater lab.
- Unreal features serpentine humanoids who can attack you both on land and in water.
- Banjo-Kazooie: The first game has only one aquatic enemy, but it's noteworthy. The Snacker is a blue shark that appears into scene whenever Banjo and Kazooie are swimming in the waters of Treasure Trove Cove (the only safe part is the water surrounding Captain Bubbler's ship). Snacker returns in Rusty Bucket Bay, guarding the southwest waters (the one with a buoy where a yellow Jinjo is).
- Banjo-Tooie: There are many in the underwater realm of Jolly Roger's Lagoon, including creatures so large that can only be stunned temporarily (such as the Blubbul and the Inky). There's also a Helpful Mook (the Seemee, a transparent fish) that doesn't attack the characters; instead, it swims peacefully while keeping an item inside its belly. There are six specimens, of which three have a valuable object: One guards a Cheato page, one guards an Empty Honeycomb, and one guards a Jiggy; Kazooie can use the Talon Torpedo ability to take the items without killing the Seemees.
- Commander Keen: The fourth episode has the only underwater level in the game, so its aquatic bestiary makes it stand out: Underwater Mines (self-explanatory), Sprites (white Atlantean-like creatures that shoot energy beams from their tridents), and the massive Dopefish (a large, overgrown green fish capable of eating Keen in obe bite).
- Donkey Kong Country has a great variety of water-dwelling enemies, resembling piranhas, sharks, clams, octopi, stingrays and other aquatic life — Bitesizes, Chomps, Clambos, Croctopus and Squidges in Donkey Kong Country; Flotsams, Lockjaws, Puftups and Shuris in Donkey Kong Country 2: Diddy's Kong Quest; Bounty Basses, Kocos, Lurchins and Nibblas in Donkey Kong Country 3: Dixie Kong's Double Trouble!; Snaggles, Snaps, Squidlies and Jellybobs in Donkey Kong Country Returns; and, finally, Finleys, Gargantuan Gills, Gordos, Fritzs, and Mama Saws in Donkey Kong Country: Tropical Freeze.
- Jazz Jackrabbit 2 has turtles on lifebuoys. Though they only appear in one level and there's about three of them.
- Mega Man (Classic): There's an underwater, scuba-geared version of the Mettaur enemy.
- In Metroid Fusion, the X can combine the forms of Space Pirates and some species of carnivorous fish to make Mer-Pirates.
- Super Mario Bros.:
- Super Mario Bros.: The game marks the debut of the most common Aquatic Mooks in the franchise, the squid-like Bloopers and fishlike Cheep Cheeps, found on almost every occasion where Mario goes underwater, alongside their assorted stronger forms. Cheep Cheeps will also often jump out of the water when Mario is passing on platforms or bridges above them. Bloopers are found almost exclusively underwater, moving either upward diagonally or downward straight, but some of them can be seen hovering in the air in Super Mario Bros.: The Lost Levels and Super Mario Maker.
- Super Mario Bros. 2: Trouters are red fish with large eyes that perform large jumps in waterfalls. By way of Cranium Ride, Mario and his friends can use them as platforms to quickly move to further areas in a level; this is exploited in World 5-1.
- Super Mario Bros. 3: Boss Bass and Big Bertha are two large Cheep Cheeps that can be found in Palmtree Panic and Under the Sea levels. The former looms around the water's surface at a great speed and can swallow Mario and Luigi in one bite, thus doubling as an Instakill Mook. The latter simply swims back and forth in a pattern as it tries to keep a very small Cheep Cheep inside its mouth (since that Cheep Cheep always tries to escape). The game also features a smaller, spiky variant of Cheep Cheeps (the Spiny Cheep Cheep) that swims the waters of World 7-4; a bigger version of it (Spike Bass) appears later in New Super Mario Bros..
- Super Mario World: Cheep Cheeps don't appear in this game, but a subspecies called Blurp does; their behavior is almost identical. The game also introduces Rip Van Fish and Porcupuffer. The former is a blue fish (apparently unrelated to the Cheep Cheeps) that sleeps peacefully until Mario or Luigi approach, in which case it wakes up and chases them relentlessly. The latter is a large purple Cheep Cheep with a spiky body that moves underwater like Rip Van Fish, but looms around the surface like Boss Bass. A secret level in the outdoors top of Vanilla Dome has dolphins, which are Helpful Mooks and completely inoffensive. Lastly, the game introduces the Fishbone, a Dem Bones version of Cheep Cheeps that charges at Mario and Luigi upon sight, which will end up killing it upon crashing against a wall, floor or ceiling.
- Super Mario 64: A cousin of Boss Bass, the Cheep Chomp, appears in Tiny-Huge Island, though strangely not in the Under the Sea levels (Jolly Roger's Lagoon, Dire Dire Docks, Wet-Dry World). They're large enough to be capable of eating Mario (and any other playable character in the game's remake) in one bite. The game also introduces Maw-Ray, a massive red manta ray that typically guards a specific part of Jolly Roger's Bay until Mario finds a way to get it out.
- Yoshi's Island features Shy Guys with inner tubes found floating around in water levels, as well as Shy Guys dressed as pirates and found on ships.
- Prinny: Can I Really Be the Hero?: The sequel has the "Undersea Volcano" as a playable stage, and aside from scuba-gear equipped Moabs (who will drown when hit with a hip-pound as their gear breaks), there are underwater Corn Men who can home on the player, Blue-colored Idols with a very apathetic outlook, invulnerable Sea Mines, and the predatory Serpent, aka the Shark Dragon.
- Water Dumples are aquatic relatives of the common Bulborb enemies, provided with fins instead of legs and adapted to aquatic environments.
- Generally, all games feature one or two enemy species that live chiefly or exclusively in the water. This can present a problem to deal with, as only Blue Pikmin can swim or survive underwater and the most powerful Pikmin types are thus useless for fighting them.
- Warcraft III: The expansion introduces the Naga, a race of Snake People, the result of magic-addicted night elves spending a few millenia underwater. They're not only capable of moving in water, they can also hide there.
- Final Fantasy: Final Fantasy, Final Fantasy III, Final Fantasy V, Final Fantasy VI, Final Fantasy VII, and Final Fantasy X which have underwater dungeons, and thus, enemies appropriate to the area (sea urchins, piranhas, octopuses, etc.)
- Monster Hunter: The third generation introduces the Sharq and the Ludroth, while the fourth introduces the Zamite. These enemies spend their time underwater, but the Ludroth and Zamite can be found on land as well. The Ludroth and the Zamite also have King Mook versions (Royal Ludroth, Zamtrios and their respective subspecies).
- Mother 3: In the underwater area, the standard Pigmask army is supplemented by Navy SQUEALs.
Shoot 'em Ups
- Raiden and Raiden Fighters series: Even though the player is relatively safe from "Sniper Tanks" over the ocean, they have to contend with their "Sniper Gunboat" aquatic counterparts.
- Time Crisis 2: The first boss is accompanied by aquatic ninjas.
- Machine Hunter: The swamp level has mutants, which assault the players (on land) with ranged projectiles.
- Warframe: Uranus missions feature underwater sections where you can get attacked by Grineer diving units armed with tridents, grenade launchers or harpoon guns.
- Mini Robot Wars has the land-based stealth unit called the Mole. In the New Game+, it gets the ability to swim on the water surface! And then you get even worse news later — the Giant also gets the ability to travel amphibiously.
"Ocean doesn't save you from Mole anymore; for it can swim now!"
- Plants vs. Zombies:
- The first game has Snorkel Zombies, Ducky Tube Zombies, and Dolphin Rider Zombies.
- Plants vs. Zombies 2: It's About Time: Pretty much every Zombie from Big Wave Beach is this, starting out from the water and eventually making it to the shore (with the exception of the Fisherman who always stays on the water).
- Plants vs. Zombies: Heroes has teammates with the "Amphibious" trait. This allows them to be played on water lanes, which are inaccessible to teammates without the trait. Plant classes specializing in Amphibious teammates are Guardian and Smarty, and Zombie classes specializing in them are Beastly and Sneaky.
- XCOM Terror From The Deep replaces the first game's aliens with a variety of aquatic aliens and altered Earth species; with the exception of the few surface-only units, all enemies are aquatic. Most of them are also able to fight on land.
- There are several mobs in the game that are water-based, such as fish, squid, turtles, axolotls and dolphins for passive or netural ones. For hostile mobs, we have Guardians and their Elder Guardians, which are only capable of using their lasers underwater. All of these take extra damage from Tridents with the Impaling enchantment.
- Drowned are aquatic versions of normal zombies. These spawn in water areas or transformed from a zombie that's submerged in water for a certain amount of time, and they're capable of swimming and staying underwater indefinitely. Furthermore, they can spawn with a Trident, which gives them a painful ranged attack. Strangely, despite being water-based mooks, they don't take extra damage from a Trident with the Impaling enchantment.
- Gundam: The various shows typically have at least one type of mass-produced underwater mobile suits for the main Gundam to fight.
- The original Mobile Suit Gundam and its spinoffs' Gog, Acguy and Z'Gok.
- Mobile Suit Zeta Gundam's Zaku Marine, which was actually an unused design from the original series that never made it into the show, brought out of retirement for a decoy operation.
- Mobile Suit Gundam ZZ's Capule, which later appears in ∀ Gundam on the Earth forces side as the Kapool.
- Mobile Fighter G Gundam's Pescatore, a Mexican submersible MS which inexplicably has a face like a parrot.
- Mobile Suit Gundam Wing's appropriately named (for once) Cancer and Pisces.
- Mobile Suit Gundam SEED's UMF-4A GO Oh N and UMF-5 Zn O, with the later UMF-3 ASH functioning as somewhere between this trope and Elite Mooks.
- Wonder Woman: During her Earth-Two days Diana fought the Neptunians whose steeds in battle that would fight all by themselves as well were giant flying fish with tentacles.
- Animorphs: One story starts when the Yeerks bioengineer gills onto their Hork-Bajir troops. It fails horribly, and the poor guys end up choking on air.