There is a dive park on site although I have not dove to that location yet. You must bring a marker as there are boats overhead. The dive shop that was there has gone out of business. (You know the joke, Obama must love small business's because he is turning large ones into small ones!) However there are two dive shops close by that you can fill your tanks up at. One is in Killeen, C&J Divers, and the other is in Temple, Blue Dolphin Divers. Both are great places and you will do fine with either. When you go to dive at Stillhouse let the park ranger know you are going to dive at the marina and you will not have to pay a park fee. Park at the parking on the curve just before the marina unless you coordinate with the marina to dive off of there, rumor has it there is a sunken boat off the marina they will let you dive at from time to time. Temps will be about the same as Lake Travis, visibility is not as good, maybe 1-3 feet but still good enough to dive in. The only thing I did not like was the boats overhead, it creeped me out, esp since I did not have a dive marker. With it you SHOULD be ok. Ah there is some sort of sea weed that grows there as well that can get to be about three or four feet tall at the bottom of parts of the lake so if you don't like that you can just swim to a different part as it is not everywhere. I would wear a wet suit because of that stuff though.
If you want to dive there let me know as I would not mind going there from time to time.