http://www.tpwd.state.tx.us/fishboat...rails/coastal/
These are coastal trails.
The San Marcos River is a place to play beneath the falls in Rio Vista park. You may also kayak from Rio Vista park or under the IH-35 bridge to CR 266. Beware of Cummings Dam, right before the take out.
If you go beyond CR 266, make sure you get a map and instructions. There are some serious (bonebreaker) rapids beyond that point.