Aren't the potential vampires of Skyrim suppose to live under frozen lakes and glaciers? I remember they were able to reach through the ice and drag there victims below.
If vampires are in Skyrim they should definately function like this. In past TES games the water has simply been a breeding ground for slaughterfish; having vampires in the water would certainly make you worry about more than just your toes being nibbled off.
Problem is that the water sections in Oblivion were so damn cloudy you could barely see past your nose. And there was literally no reason to go in the water unless you needed a shortcut, or perhaps a section of a dungeon required you to swim to the next area. I would really love to see Skyrim improve on both fronts; you know, make the water a place you'd want to visit. Improve visibility, add vegetation, some new creatures, interesting underwater caves filled with the aforementioned vampires.
I think Black Marsh would be the perfect place to implement an underwater environment though. I imagine the Argonians having cities on both land and underwater, so maybe we should just wait for TES: Argonia for some truly epic underwater stuff.