Originally posted by Samoth:

potions of waterbreathing, ftw.
The gorgeous environment would look even better if we could walk a few moment on the sea floor. And/or the possibility of surviving a tragic sea ordeal. It looks amazing down here!

I bet they had this a idea but the game being strikingly similar in visual to Skyrim they might alter some names or mechanics of the thing that you are suggesting, IF they decided to add such thing into the game that is.