There are two outdoor pools in the Valley, one at Yosemite Lodge and one at Curry Village. You can also swim and paddle in the Merced River which runs through the Valley. I find it a little cold but it is a great way to cool off hot, tired feet.
If you fancy floating down the river you can rent a raft at Curry Village. This is great fun especially as you do not have to do any tiring paddling back but just get out at the designated end point and meet the shuttle to take you and your raft back to Curry Village.