Definitely a lot of sea options.
Something along the lines of a swimming termites (Some sort of name that goes along with that?). Boats are made of would right? It could be a dragon unit or ability, they could cast it and it would slowly do damage to the ship, but if the ship boarded another the termites would pass over to the other one as well.
A sea creature that can make a tidal wave would be cool, and fun to use, but maybe to much of an annoyance to combat. At the moment I can think of nothing too specific that isn't obvious, like tentacle creatures, ect.