A small space heater with an Anti-freeze thermostat is also cheap insurance. (provided it's not going to burn your boat down.)
I keep my boat, un-winterized, in an insulated garage. I keep a space heater in the ski-locker set to 42 degrees. Plus when it gets really cold, I keep a 200 watt block heater on the engine plugged into a different power circuit.
This year, I'm thinking about adding a temperature data-logger/alarm.