Yep most everything down here is on the Harris County Regional trunked system:
Scanner Frequencies: Harris County Regional Radio System (formerly STAR-Net) Motorola Trunking System, Metro Houston area, Texas
If you'll just be on the island you only need to worry about tower site 19. Are you coming down for the sandcastle competition?
Make sure to program in the three Beach Patrol TGs, plus Galveston fire, EMS 1 & 2 and PD will be busy I'm sure. Don't bother with the constable TGs, they are fairly quiet. If you want to listen to beach action on Bolivar program in GSO 1, 2 and 3 as well as Bolivar Fire. Don't forget the Countywide Fire TG which is used for ground contact on helo landings.
Edit: Forgot to mention the Park Board TGs if you are interested in the administrative, parking and some PD type activity specifically related to Stewart Beach and East Beach. Also the Galveston County Event channels *might* be used if the sand castle deal is big enough.