This was originally a 2D entry, but I changed to 3D because I figured this might be a good opportunity to practice my modelling and texturing.
Here are my sketches for my underwater warrior:
I like to think that his head is a kind of lantern. While the large part of the body is a giant shell, he has large shields shaped like claws protecting his arms and feet, which he can use to push his enemies backwards. His tiny claws are used to grip on to his weapon, which I am thinking would be a spear, since it looks like the easiest to wield underwater.
The light-bulb like part dangling from his head could be used to blind and hypnotize enemies.