Google "ScatMat" or try PetSafe.com. A few harmless but painful electric shocks might do the trick. Be glad you're not in Newport Beach, Ca. dealing with sea lions!
We have a scatmat at home, I may try that but I did not think it was water proof.
Plan B, Coyote urine, a lot of coyote's around here, the River Otter may take notice of this. And the Coyote is listed as a natural predator to the River Otter.