There is a spell on this site for that:
But, I personally would NOT try it in the case that there is a possibility that this spell was added by a Fluffy. It could be potentially dangerous for the fact that something could go wrong, because all it takes is one simple inhalation of water, and you could die. Also, you may not know when the spell will wear off.
You can not breath underwater like a fish, as you have no gills and before you ask, no you can't grow gills either. You can accomplish this by using a scuba tank or you may travel underwater using astral projection.