I have the Boatsafe 1000 watt heater. It's great for keeping the engine compartment warm. However, it isn't going to do anything for anything forward of the engine room bulkhead. Water lines that travel forward still need to be drained.
My boat sits on a trailer in my driveway, so if I lose power I know about it pretty quick. Assuming your boat is slipped, if the marina loses power, or somebody unplugs your boat, you do run the risk of freeze damage. In other words, I recommend winterizing the boat and using the heater as insurance.
