"May already be mentioned by others, but a boat with only electric heat would be a non starter for a lot of potential buyers."
A diesel heater with fans ,pumps, and juice required to operate is still locked into the power pole long term .
If someone is cruising in cold waters , or winter operating this will be a big factor in heat decisions.
FF is right on re the Espar and Webasto forced air units. Their hydronic units may be better, but only the Dickinson style stoves and heaters can get away with negligible 12v consumption and zero AC consumption.