I've tried owls, snakes, scary looking balloons, none worked very long, the birds get used to them. Stringing wire or fishline over the spreaders, masthead, radome, bow and stern pulpits, dodger and along the boom has been effective for cormorants, ospreys and gulls. The small birds are aren't deterred but not that much of a problem, their poop turns to dust in the sun.
Cormorants are the worst.