Better water graphics would be awesome... as far as swimming goes I highly doubt it..

In GW2 they have 2 different sets of weapons, one for land and one for underwater. I also believe they have an entirely different set of abilities when they are swimming as with the monsters that can swim, they change their style of attack and abilities as well..

From what I gather they are focused on fixing the regular combat, doing something underwater from the start would be extremely difficult and highly unlikely