Rubber snakes were recommended to me over owl decoys. I throw 'snakey' on the deck every time i disembark now, seems to keep the droppings to a minimum, especially compared to some of my mooring field neighbors. One had Canada geese nest in his cockpit and couldnt sail all spring.
Doesnt deter the little birds that like to ride the wind indicator around like a carousel but their business is small time. So far I've avoided the sea otter"s playful destruction too.
I think the big wiggly ones work better than the coiled ones, and avoid exotic color patterns.
I have not seen anyone succeed very well with the plastic dinosaurs but they"re pretty funny.