I useed a mid grade alarm ( forget the brand?), and installed a shock sensor (someone jumps in the boat it goes off ( or trys to steal my prop). I put pin switches on all my compartments. If they open one the alarm goes off. The motion sensors are worthless on a boat, they give alot of false alarms.
Prior to all this I used to have a baby monitor in my boat. You cna hear anything around your boat. Even people whispering , is loud enough to wake you.
My alarm has scared away some would be theifs trying to break in.