Depends on how much heat you want, but if you are looking for just something to prevent freezing you can look at a gun safe heater. Much cheaper than the option listed above, very safe, but low heat output (12-24W depending on length). Either plug it in and forget about it, or wire it to a thermostat outlet.
The draw is low enough that I'd just run it all season. If its a big bilge I wouldn't hesitate to run two