Water units

Triton, Naga, Dragon Turtle, Ghost Ship, Water Weird, Nyad, Nixie (maybe...depends on your mythology), white worm.

From your list of sea creatures, I like Kraken and wouldn't blink twice at Giant Squid.

Wendigo is a good ice one. Oozes can be fun for water.

You could always just create new creatures out of other environments: most of the ones in the list are palette swaps of more generic monsters (water + elemental, blue + dragon, mer + man, frost + giant). You could have awesome icicle ninjas who throw icicles rather than shuriken, and then dissolve into a freezing mist for stealthy travel. You could make water vampires who suck the moisture out of things for sustenance (maybe they have limited "bloodbending" powers as well). Selkies are a good were-watercreature. You could make a starfish that can superheat water to travel around on a jet of bubbles underwater. You could make water zombies where the infection caused you to slowly melt away, giving clayface style powers for a time. You could make a water demon who summons bubble bombs out of ambient moisture that burst with a strong concussive force: he would look like a normal human except his eyes would be bubbles. There's a lot of possibilities: obviously you want to rename so it's not an obvious skin, and it's not going to be as resonant, but you get the benefit of something surprising and new.