I've lived aboard in NE at dockside with an espar and a small 110v ceramic heater.
A clear shrink wrap cover adds a lot of heat... and it doesn't have to be completely sealed. In fact you WANT vent holes.
People at my winter layup who live aboard use some sort of propane heaters and get a large tank which sits on the dock (safer no?). All have plastic covers.
Core hulls are reasonably well insulated, but you want to cycle in fresh air and dehumidify it as well. Water in the winter is often warmer than the air except on the coldest days.
The trouble is the ice on the boat (if uncovered) and the docks. This is really dangerous.