Jay,
If you can find some clear hill country streams you might find some softshells, Texas Map Turtles, Texas River Cooters moving around on the bottom.
Almost anywhere, you should be able to spot some turtles basking on logs, etc, if we get some decent sunshine and temps up in the lower 60s, however most of those will probably be Trachemys.
The San Marcos river used to be (and may still be) a great place to see turtles by snorkelling. We used to just pull into some of the parks in the city, and snorkel in the river there. The water temp is a constant 72° so you can snorkel in the winter, but getting out can be damn cold! I don't know the legalities of doing this anymore, but people swim/tube there so I can't imagine there are restrictions.
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Chris Harrison
San Antonio, Texas