Giant Mantis Shrimp.
Look that fellow up on TED Talks and tell me you don't want to see a creature that can hit with supersonic jabs equivalent to a .22 caliber bullet, punching something so hard it
glows for nanoseconds due to some crazy science thing like the water around the punch being vaporised and heated up to the temperature of the sun, incidentally causing shockwaves to follow split seconds afterwards and smash open whatever it hit. I'm not even making this up, look it up. The shockwave apparantly stuns or kills you even if it misses- it has
supercavitating fists.
Or supersonic spear-thrusts depending upon the type. Giant so they don't just mangle thumbs of dwarven fishermen and crush snails but actually climb onto land and start crushing people's chests, since in the real world sadly they're too small to be dangerous to large animals.
Other interesting info: Their eyes are the best eyes ever. They have compound eyes divided into layers, each layer specialising in a certain thing like "Ultraviolet Light" "Polarised Light" "Visual Spectrum", etc. Their eyes constantly swivel independently on stalks in order to scan different layers over the range of their vision, especially as they only need 1 eye to see with Depth Perception unlike humans.
If you are there they can see you and will come for you. It'd be the aquatic GCS except it'd have way cooler ways to die. I'd go out of my way to fight them in adventure mode.