Yoshi himself have talked about possible swimming.

As for swimming, it would to be something that wasn’t just swimming. If we were going to put this in the game, we would want to make it a part of the game. For example, swimming to certain dungeons to do things, battles in water, etc. While this isn’t something we could put in for 2.0, it might be something we consider for the next expansion pack. We’d make sure that it would have something worthwhile.
We do have the swim suits, so they have to be good for something, right?
http://pc.rpgsite.net/interviews/327...-naoki-yoshida