We're heading full-tilt towards another (nearly annual event, at this point) Texas drought, so your options will narrow as the summer wears on. Other than that, any place with water affords plenty of opportunities for swimming and most, as long as there's enough water, for jumping/diving in. For waters within Texas State Parks, the Texas Parks & Wildlife Department monitors water quality and will post warnings and, if necessary, close off swimming spots when the water quality falls below acceptable levels. So, go out and have fun.
We've got lots of lakes, lots of rivers and plenty of fun to be had. Justin and I just got back from 2 nights camping at
Guadalupe River State Park, and I hear
Inks Lake State Park is doing great as well.
Bear in mind anywhere with water will be EXTREMELY crowded because it's hot outside and everyone else is thinking "How can I cool off and let the kids play at the same time?" I had never before seen Guadalupe River State Park as busy as it was last Saturday -- I was shocked. So, just be prepared and get to wherever you choose to go early so you can pick a good picnic spot or shaded area to retreat to.