When I was painting up all those pirates and swashbucklers over the summer, I also tried to pull together all the aquatic monsters in my colleciton. It turns out there weren’t all that many unless I count plastic and rubber animals which could be giant crabs, octopodes, sharks, and that sort of thing. Here’s what I did have to paint.
A Reaper marsh troll, a TSR scrag, and Grenadier sahuagin. I spent the most time on the sahuagin, since he’s got a lot of gear, and the Monster Manual description mentions that they are lighter in front, and dark in the back, which makes sense for a fish-man; really almost all animals are lighter on their bellies/undersides and darker on top, as camouflage in the water. I think land animals might retain that as a leftover from evolution. The eyes are described as shiny and black (dead eyes like a doll?) so I just put a couple of dots of white to suggest a glint. The troll I painted similarly. I believe I read that sahuagin leaders are larger, so the marsh troll is especially suitable on that count, though he should maybe have some equipment.
Before doing those guys, I also painted a pair of mermaids (clear plastic drink hangers), a merman (some kind of rubbery gumball machine toy) and another scrag.
Not sea-related but I also repainted a MageKnight figure that I assume is some kind of night hag on a nightmare. I think that’s some kind of organ she’s hurling. A little big for a heart … maybe it was supposed to be a fireball?
Swords & Dorkery 
Sweet