I’ve seen rubber snakes used on a few boats if I recall in Bridlington harbour so does seem to be a thingOne of my sailing mates uses rubber snakes spread about the coachroof and foredeck. Seem very effective, although it amazes me that seabirds come hard wired to avoid a species they presumably never see. The things are about two feet long and brightly coloured, and are kept in place beneath deck lines.
I use Gull Sweeps on my dory, which is close to the hammerhead jetty in Mersea, and used by bacon-addicted gulls as a launchpad to harvest the windfall from the trippers' crabbing lines. They're pretty effective, but birds soon learn to perch and defecate beyond the radius.