I lived in Houston for several years and had St Augustine there. I still have friends and family in Houston, and I have heard from several of them that the drought has destroyed their grass and they are figuring that they will have to resod it. They are waiting to see if the drought breaks before they do anything, since a new grass will not make it in these conditions.
It sounds like you did what you could, and that is what you should have done, but I don't know just how dry it was and how far the water actually soaked into the ground. If it did not get below an inch into the soil, you may have caused all the roots to be too close to the surface, which made it more possible for it to dry out the roots and kill the grass. If the conditions were as dry as I have heard, I would think you may have needed to water longer at one time. But if that is the case, it is too late for that.
One good thing is that St Augustine is pretty tough, and if you still have good grass, the runners may spread into the dead areas if they are not too big and if you get rain before it all dies. That may be a tough call with no break in the drought. Just know that you are not alone---I am surprised that you have any grass left from what I have been hearing. You may want to start saving money to have it resodded if you ever get a lot of rain.
The good news is, you don't have to mow dead grass. 
Seriously though, your next problem will be keeping weeds from taking over in those dead spots---somehow they can still grow! Sorry I don't have better news or more help.
Edit history
Recommendations
0 members have recommended this reply (displayed in chronological order):
= new reply since forum marked as read