Our boat came to us equipped with reverse cycle air conditioning units. They work well keeping the boat comfortable down to about 0C. Something to be aware of with this kind of unit, it that it does not work when the water temperatures drop below about 5C. They are quite energy efficient, actually using a bit less power than they do when cooling. They each deliver about 2,000 Watts of heat while using a bit less than 900 Watts of electricity.
Amel also equipped our boat with resistance heaters, one in each cabin. They draw a lot of power, if they were all running about 20 Amps, more than the shore power connection could support.
The boat is in Charleston, we are finishing up a delivery job in Jolly Harbor, Antigua