If lake swimming is important, there's Keyhole State Park just NE of Moorcroft, WY (Devils Tower area). If we have a dry winter and spring here the sandy beach is nice; if not there won't be much beach. Elevation is somewhere around 4K feet, so it does warm up enough for swimming. We used to water ski there regularly without wetsuits, and when my son was in high school he and his buddies would skip school to go swimming and diving off the cliffs.
(That worked until the cops caught them with their beer. Everyone else took off, but my son, being a friendly chap, walked up to the cops and offered them a couple beers. They weren't friendly back.):h
Keyhole is a popular spot with the locals, mostly for fishing and boating. There are plenty of camping spots during the week, but my wife and I prefer camping in the Bighorns or Devils Tower. The Bighorns have plenty of lakes, but the water is cold, cold, cold. Good for trout, bad for swimming.
