Aquatic Mook

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 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 in 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 out of it (if they can be damaged that is). Some games (like Mario) may have the same enemy show up in both roles, others will have different ones for the two environments.

Compare Gas Mask Mooks; a Sub-Trope of Faceless Goons.

Examples of Aquatic Mook include:

Video Games

There's an underwater version of the Mettaur, which has scuba gear and stuff.
There is this kind of Genesis Clone in Crisis Core, namely the ones which Zack beats with a parasol.
The underwater level of The Operative: No One Lives Forever features a lot of these.
Golden Eye 1997 and The World Is Not Enough's video game adaptations
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.
The standard Pigmask army is supplimented by Navy SQUEALs in the underwater area of Mother 3.
Snorkel Zombies and Ducky Tube Zombies in Plants vs. Zombies.
Final Fantasys I, III, V, VI, VII, and X have underwater dungeons, and thus, enemies appropriate to the area (sea urchins, pirahnas, octopuses, etc.)
Jazz Jackrabbit 2 has turtles on lifebuoys. Though they only appear in one level and there's about three of them.
X-COM: 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.
Yoshi's Island features a Shy Guy with an inner tube as well as Shy Guys dressed as pirates.
The arcade version of Contra has two of these that jump out of the water at the beginning of the game, and nowhere else.
The first boss in Time Crisis 2 is accompanied by aquatic ninjas.
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...