Well, I live about 2 hours south of Houston and have heard of a few places to dive close to Austin area. You have Lake Travis which I hear is quite good. Also, if you travel south for about 30 to 40 minutes, you can run into some Rivers such as the Comal and the San Marcos or even Canyon Lake. All of these are greater than 4 hours for me, so I have started looking toward rig diving or driving 4 hours the other way to Toledo Bend (where I spent alot of time growing up at). It does help that I have a family of divers who still live within an hour away from Toledo Bend, so it makes the dive trip a family visit also.
If you get into diving and preserving nature, I would suggest the Scientific OW diving course (around $300) and head down to Aquarena Springs. It is very clear (about 40-50 ft vis) and a constant 70 degrees almost all year round. With the Scientific OW certification, you can dive there for free and help them out a bit by helping cleanup. Pretty awesome deal when you normally have to pay $25 per day to dive in the restricted area.
