[Wrana]

Actually, Earth being about 75% water-covered doesn't mean anything for fantasy universe. That's the same as most fantasy authors place their adventures in the North hemisphere of their worlds. Just an economy of thought (sp?). Actually, fantasy world can have almost no seas and even be a Mars-like desert one.
What I agree with is that more aquatic nations would be fun - but I guess that there are just not much good ideas of such in the developers' minds at the moment. And I don't remember much mod nations which are aquatic, too... It's possible that some Polynesian folklore would be helpful, but I don't know this enough...