We have been there a few times. The park is on the Frio river.
They have an old section and a newer section. The old section is close to the water and pavillion but gets packed out. The newer section is a little further but more spacious.
Reservations can be hard to come by.
Check out South LLano State Park. We liked it better but it is further from the water...www.wildtexas.com has a lot of info.