I've come across this issue as well when mooring past boats on a mooring buoy in front of our beach cabin. The neighbor has a GW and uses canvas covers, an owl and the things that twirl around... none of which work too terribly well. I think I still have a picture somewhere on my phone when I was rowing my boat back in and saw a seagull sitting next to the owl on his boat near the outboard. As stated above, they're very smart and with something that doesn't move they know it's not a threat. I've seen at ferry terminals things that look like porcupine quills on small piece of metal that extend out so that the birds can't sit on top of them. They work pretty well on the pilings and so forth. That being said, I'm not sure you'd want them on a dock.