Your basic Mook, now for aquatic environments. Either they will be equipped with scuba gear (masks and air tanks), become more fish-like, etc. You'll definitely encounter them Down the Drain or even in Bubblegloop Swamp. This most commonly occurs with the Mascot Mook.
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.
- The Ecco the Dolphin series is packed with hordes of these, given that a lot of it takes place under the ocean.
- The arcade version of Contra 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 will sometimes return to the water to replace it.
First Person Shooter
- Deus Ex has a few Diver enemies on the way back from the underwater lab.
- GoldenEye (1997) and The World Is Not Enough's video game adaptations.
- The underwater level of The Operative: No One Lives Forever features a lot of these.
- Unreal I features serpentine humanoids who can attack you both on land and in water.
- In stage 3-2 of 8Bit Killer, you encounter gun-toting enemies who use their diving suits to submerge in water to catch you by surprise.
- The Donkey Kong Country games have a great variety of water-dwelling enemies, resembling piranhas, sharks, clams, octopi, stingrays... Bitesizes, Chomps, Clambos, Croctopus and Squidges in the original game; 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.
- There's an underwater version of the Mettaur, which has scuba gear and stuff.
- In Metroid Fusion, the X seem to be able to combine the forms of Space Pirates and some species of carnivorous fish to make Mer-Pirates.
- Yoshi's Island features a Shy Guy with an inner tube as well as Shy Guys dressed as pirates.
Real Time Strategy
- 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.
Role Playing Game
- There is this kind of Genesis Clone in Crisis Core, namely the ones which Zack beats with a parasol.
- In the Final Fantasy series, there is 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, pirahnas, octopuses, etc.)
- In the underwater area of MOTHER 3, the standard Pigmask army is supplemented by Navy SQUEALs.
- 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.
- The first boss in Time Crisis 2 is accompanied by aquatic ninjas.
Third Person Shooter
- Uranus missions in Warframe 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!
"Ocean doesn't save you from Mole anymore; for it can swim now!"
- And then you get even worse news later- the Giant also gets the ability to travel Amphibiously.
- Snorkel Zombies, Ducky Tube Zombies, and Dolphin Rider Zombies in Plants vs. Zombies.
- Pretty much every Zombie from Big Wave Beach in Plants vs. Zombies 2: It's About Time 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).
Turn Based Strategy
- XCOM Terror From The Deep replaces the first game's aliens with a variety of aquatic aliens and altered Earth species. Most of them are also able to fight on land.
- 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.
Wide Open Sandbox
- Minecraft has Drowned, aquatic versions of normal zombies. These are spawned in water areas or transformed from a zombie that's submerged in water for a bit of time, and they are capable of swimming and staying underwater indefinitely. Furthermore, they can spawn with a Trident, which gives them a painful ranged attack.
- The various Gundam 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.
- One Animorphs story starts when the Yeerks bioengineer gills onto their Hork-Bajir troops. It fails horribly, and the poor guys end up choking on air.