Yup, that's a tough one. The only "warm" water I know of in Texas that time of year will be the Comal River and Balmorhea State Park (both in the low 70's year round). Unfortunately, both are quite shallow, so if you are taking them for checkout dives, that might be a problem. Balmorhea is a little deeper (max of about 20 feet...if you stretch), but it's about as far away from you as Morrison Springs or Vortex Springs in Florida. The Comal, in New Braunfels, is a nice drift dive but, again, 18-20 feet is about the max depth. I cannot think of a single lake that would be anywhere near what I would call "comfortable" that time of year.
If you find an answer to this one, please let me know!