Well there is said to be glaciers in Skyrim, and I would put them to the north. If we get arctic like glaciers in the water with sunken ice caves, I think we could be in for some really beautiful lighting effects if the water is clear enough. Unless I'm ill prepared for it at the start due cold, I think that would be one of my first stops.

So the area seen in the gameplay trailer is near one of the lakes?

I definitely want there to be underwater life in some places, though there should also be a lot of ponds and small lakes where you can swim without fear of being attacked. In the ocean there should be some sort of legendary sea serpent that is rare but next to impossible to beat. That sort of epic sea monster battle definitely has a "Conan" feel to it, so would work with Skyrim well.