Yeah I'm not crazy on that idea...I guess physically it could be hard to swing a sword underwater with enough force to kill stuff??? I dunno. But I stumbled on that unknowingly. Ooh there's a slaughterfish...swim swim swim .... oh wtf, can't draw my sword!? I ended up waiting on the shore for it to surface and lightning-ed at it.
Also for some strange reason the flame spell can go through water, I just killed a slaughterfish this way..blindly aiming into the water. I think this is a silly way to do this...Don't know why bethesda thought this would fly