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 a lot of it 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 I features serpentine humanoids who can attack you both on land and in water.
- 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.:
- The most common Aquatic Mooks in the franchise are 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.
- 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.
- 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 I, 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.)
- 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.
- 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.
- Minecraft has Drowned, 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.
- 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.
- AnimorphsL 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.