A decent car alarm with a couple well placed proximity censors and a couple of 130db "paint peeler" alarm horns.
After I had half of my tower speakers stolen right out of my backyard a couple of years ago, I installed such a system. Now anytime anything moves within two feet above or around my boat, the alarm warns them. If the system detects movement again, the alarms will go off defening anything close.
These days, birds, neighbor cats, and chipmunks stay well clear of my boat.
It's also fun to see people walk up on it in a parking lot, and have it start chirping and warning them. People just don't expect an alarm on a boat.