The reverse cycle air conditioner is much more efficient than the space heater because it extracts "free" heat from the ocean. In reverse cycle, heat pumps work by sucking in, say 55 degree water and returning 50 degree water over the side. That five degree drop is a lot of heat energy.
In
HVAC equipment the ratio of thermal energy produced divided by the
electrical energy consumed is called COP (Coefficient of Efficiency). By definition, a resistive heater like an
electric space heater has a COP of 1.
A high efficiency gas furnace has a COP of about 2.
A typical air source heat pump like a home air conditioner has a COP around 3.
A water to air heat pump (which is what are in our boats) has a COP approaching 5 (it's actually not quite this good as the small
water pump electricity is not included)
So our boat AC systems produce almost four times the heat energy for the electricity as does a space heater. The great efficiency is because water contains much more heat energy than air - and we have an unlimited supply of water.
But the efficiency of standard boat heat pumps do fall as the water temperature falls due to design limitations with the
compressor. Starting around a water temperature of 50 degrees they will start putting out substantially less heat and at 40 degrees put out little to no heat.
Air source heat pump compressors for buildings have recently been improved so that the best will heat down to -20F outside air temperature.
Massachusetts (and some other northern states) are now offering large rebates to encourage people to switch their home heat from oil furnaces to Air Source heat pumps because they save lots of
money in the long run as well as greatly reducing CO2 emissions.
It's false economy to "save wear" on the compressor. Compressors will run just about forever. The part that fails are the water pumps or
leaks in the refrigerant lines.
Cheap electric space heaters, on the other hand, rarely last more than about six months of constant use - often failing by catching fire (the oil ones last a good bit longer but the Chinese undersized cords are prone to overheat if the unit is run at the full 15amp
power ). Most
marine insurance companies prohibit the use of portable space heaters because of this fire risk and will deny a claim if a boat fire is caused by one.