Fish can drown under certain conditions.  It is not the water that keeps fish alive, but the oxygen in the water.  Fish breathe oxygen, just as we do.  The water contains oxygen that comes from the air or is given off by water plants.

A fish takes the oxygen out of the water by means of its gills.  When a fish is placed in a small tank of water, however, it may soon use up all the oxygen that is in the water.

If this happens, the fish suffocates.  This is why, if you keep pet fish, you must change the water frequently in order to give them a fresh supply of oxygen.–Dick Rogers