Could be like Sea in XI, you know, where you didn't swim. It was like walking/running normally but everything had an underwater feel/look to it.

While I have actually enjoyed the underwater mechanics of games like GW2, because it actually gave the impression of my character really being underwater. I mean really, when you are in water of any kind about the only thing you can do fluidly is swim. So yeah.. fighting in the water would be slow somewhat in reality, and clunky because.. it's effing water. It slows you down, weighs you down (if you're wearing clothes other than a bathing suit or simple/light clothing).

That being said, I would like to see an underwater area, but honestly I don't think I'd want to swim through it. While it might be slightly immersion breaking, since this is a fantasy realm who's to say someone can't cook up some magic or magic device that lets us walk/run, breath and even talk normally while underwater.