I don't have the spec's but if you figure a heat pump needs to absorb between 10 and 15 degrees of heat from the water and therefore the refrigerant temp needs to be about 10 degrees below that, you will ice up the heat exchanger
. What temperature does seawater freeze at?
We do closed loop ground water heat pumps regularly and they pretty much become ineffective when the ground temperature falls below 25 degrees. The water temp going out to the loop is only 5 to 10 degrees.
In any case, you would have to run the generator
to run the heat pump, burning diesel
that would be more efficiently used with a diesel
heater. I'd also guess that since the air will be hotter from the diesel heater it will also dehumidify the boat better.