Well, there's Lake Travis, Canyon Lake, Aquarena Springs, San Marcos River, Comal River... And that's just between Austin and S.A. Aquarena Springs requires an environmental certification, but if you plan to dive there plenty (if you're in town a while) it might be worth the 220 one time fee. Lake Travis offers you deep diving, along with dive parks to go tinker around in. Canyon Lake, from what I hear, is 80' max depth along with 5'ish viz, correct me someone if I'm wrong. Comal and San Marcos Rivers offer clearer drift diving, and often times small treasures from the drunk tubers. You might also be able to do a day trip to Balmorrhea or Lake Amistad. Those are both in West Texas, and are supposed to be good diving...