Tubing in San Marcos, Texas is almost a religion here. If you aren’t on the river then there is something wrong with you. Or, you just have a really bad sunburn. Located in Hill Country, Texas State University is a beautiful campus with a river that runs right through it. Check out the wikipedia article on it here.
The water is the same temperature year round that is perfect on a nice summer day. If you want more diversity, drive down to New Braunfels, Tx and check out Schlitterbahn Water Resort. Or, if you want to stick to the rivers or lakes there are tons of choices including Lake Travis, Lake Austin the Blanco and Guadalupe Rivers.