We are moored in salt water under a covered slip. We also use our boat year round. Anything that's within about 6" above the water line and below, won't freeze. In the Pacific NW, our waters stay around 43 degrees.
I installed a Pali engine/bilge heater (400 watts) in the cockpit. It is permanently mounted (screwed in). It plugs into the BBQ grill outlet. It's thermostat is hard coded to keep the temp above 40 degrees.