Napa auto parts sells UL rated magnetic block heaters. If I remember right they are $40.
I use my boat all winter. After pulling it out I drain the block and heater (takes minutes). I then put the heater on the side of motor. When I check it, you feel the air under the engine cover is warm. The engine is slightly warm too.
In addition to the block heater, I leave a "Golden Rod" heater in my rear ski locker (the shower is back there) and a dehumidifier in the boat. When I take the cover off, the air is slightly warm and dry.
I use the block heater instead of lights, so I don't have to worry a bulb going out.