When in port I run a small flow of HOT water continually back to the main water tank .... water is heated by 120v hot water heater and it becomes quite a 'heat sink' that continually emits heat into the boat at a constant rate ... for up to 8 hours. Obviously you dont want to have the tank water exceed 180 deg. so you dont soften the 'plastic' components. Best of all you get less 'condensation' as with 'fired' systems. You can 'store' a lot of BTUs in a 90 gallon water tank. Most water heaters are 1500 watts and that translates to about 5200 BTU per hour (not including losses). .... about 3/4 of what a typical reversing heat pump can do.
eg. (140-60°) X 90 gallons X 8.33 lb./gallon = ~60000 BTU Only problem is it takes almost a full day to get the tank up past 130 deg. but once stabilized its fairly easy to maintain the temp.

When traveling (by motor), I do the same thing ... run engine heated water back into the main water tank.
I use either a catalytic heater or the propane stove to 'top off' the chill.

Sure needed to do this last 2 winters in the Keys and the Bahamas.