These photos are grainy, but in the original video the animal is clearly and indisputably an otter. Clear shots of tail and typical otter movements in the water.
Photos are from a video recorded by a park visitor, Michael Procopio, and used with his permission.
Close up photo includes one shell and two valves. Group photo includes all smooth pimple back at that survey site.
County record captured by myself and Wes Anderson. This individual is now specimen 74704 at the herpetology collection at University of Texas at Austin. It was a gravid female.