I voted yes, i loved the fight with the giant Slaughterfish in Oblivion. I always think about deep water and giant, horrific creatures living in the dark water. Felt odd to go into deep water and still only be attacked by wimpy slaughterfish. Maybe if there were huge fish to fight, maybe a squid like creature (kraken?) or some type of other creature.
I voted no. I don`t have anything against underwater fights but Skyrim is a cold place. You`d only expect smaller fish to be there. And maybe a whale.
What about the "midnight zone" in the darkest depths of the ocean, like the angler fish or those bio-luminescent jellyfish. (ok maybe jellyfish arent that scary, but it would be cool to see them)
There could always be hydrothermal vents in some of the caves to explain how they could grow large in cold temperatures. Maybe even a creature that is not solely aquatic, maybe some type of dragon that spends time in the water?