In Oblivion, the waters were virtually pointless, literally nothing in them to fight except the RARE slaughterfish.
In Skyrim, the northern coast should be full of ridiculous creatures that we should be scared of. I mean, I'm not going to name anything specifically because there could be any number of cool things they could do, I just implore Bethesda to please make the waters SCARY! Deadly! Not just some pond with goldfish in it! It's the coast of Skyrim, I think it would be fitting to throw in some old Nordic/Celtic/Viking mythological creatures in there. It'd be really fun.