Bill, I suppose you know by now, but here goes...
A normal A/C unit uses outside air or sea water to cool the condensing coils. A heat pump can be reversed... taking heat out of the water and releasing it into the furnace A/C plenum. The colder the water, the less efficient is the heating and the more efficient is the A/C. High power consumption is due to the need for a compressor to run either direction. Safety factor is high since no flammable liquid/gas is needed, but still very inefficient heat.
Lots of trawlers use those units for heat and A/C, but you have to run the generator to run the compressor for both heat and A/C.
John