South of San Marcos-
Canyon Lake is an option nothing special about it, engineered lake. Lots of cars in one end.
San Marcos River-
Might want to add the San Marcos River- that is a nice float/drift dive in San Marcos.
I have developed a simple in/ out routine for people driving in a single car.
Aquarena Springs-
The scientific diver course can be done and then you could dive in the protected Spring lake of Aquarena.
Web-
http://www.aquarena.txstate.edu/diving.html (scientific diver course.)
Sidenote- the San Marcos river is spring fed, clear and a constant temp all year round- but not deep. Great in the winter.
Balmorhea State Park-
Balmorhea State Park is a good winter site. Out by Pecos Texas (heck of a drive) the site is a converted spring into a very big swimming pool. It is fun for about one weekend in the winter when you are feening for a warm water dive. Be warned the town is basic- as in two dining options. If doing this I would suggest hitting the observatory or Carlsbad cavers. We did Carlsbad and had a blast!
Interesting Map from Dive Training-
http://www.dtmag.com/dive-usa/sc.html
However, be warned they are numerous errors on this map for names, and locations.
But it is a place to start.
In the future-
Rumor has it that A&M is to sink there older ship some where in South Texas. Yet, to hear specifics about that; more to come.
The USS Oriskany (ex- US aircraft carrier) is to be sunk off of Pensacola Fl. and I am already looking to put together a group of divers for a weekend trip. 12hour drive (max.) it would be probably a three day trip. Leave Friday early return Sunday. The site is a far ways out (25 miles off shore- off the top of my head.) The outing is for advanced divers only! Flight deck will be an estimated 100ft and lower flight deck is at an estimated 130ft. Overhead environment and great site for wreck, photo, and decompression.
Will advice as things develop