Among all of the reservoirs that are great for swimming in the long warm season Saint George offers, there are also three public swimming pools with slides, deep ends, and shallow ends. There is also an outdoor pool in the nearby town of Veyo. Please follow the links for their specific seasons, directions and prices.
City of Saint George:
http://www.sgcity.org/departments/recreation/swimming.php
http://www.sgcity.org/departments/recreation/swimming.php#tab-2
City of Washington:
http://new.washingtoncity.org/communitycenter/
Veyo Pool Resort:
Just 17 miles north of Saint George is a local secret: Veyo Pool Resort. This is a great place to swim that also offers camping, shaded picnic sites, hiking, hot burgers, fries, etc. and even rock climbing.