The Fling is a liveaboard. Leave Friday night and get back Sunday. Too far out for day trips. For our time and money, we go to Coz.
As an example, Tom's has a group trip to Coz that runs $1100 for five days with 6 dives (three of us are going in March). The diving is better, you have a better place to sleep, and while the food on the Fling is good, Coz has much better options. On a final note, you are a ferry ride and a hours drive from the cenotes.
Your best option for one day is with Robert at Lake Travis Scuba. There actually are some interesting places to dive in the lake. During the summer, there are large party boats that anchor around Starnes Island. Lot of good treasure dives for goodies dropped by the drunks. Both the Comal and San Marcos Rivers are spring fed and maintain an essentially constant temp in the low 70's. Between the two, Comal is the better dive. Drift diving and it works best with a group of two or more. You do not want to walk back to your car at the end of the dive.