Starcruiser,
I hate to rain on anyone's parade. And, I know these systems and especially the physics are complicated. But, the other folks do have it right. Typical residential heat pumps do move kinetic energy (manifested as heat) between the interior and exterior of a house, and vice versa. And, typical marine units do move kinetic energy (manifested as heat) between the water and the cabin.
Something which has no kinetic energy has a temperature of 0 Kelvin, -273.15 degrees Centigrade, -459.67 degrees Fahrenheit. At temperatures higher than this there is kinetic energy -- even if cold by human standards. And, anywhere in the widest range of temperatures where humans can exist, there is a ton of kinetic energy. 0 F is 255 K. 100 F is 311 K. There is only an 18% difference.
Heat is a form of energy, specifically kinetic energy (all heat is a form of kinetic energy, but not vice versa). And, there is a law of physics known as the "law of the conservation of energy". It basically means that energy can't be created or destroyed, just moved around or converted from one form to another. And, this is exactly what heat pumps do -- move it around.
When a heat pump is cooling, it is collecting kinetic energy indoors and moving it outdoors. This has the effect of making indoors cooler and outdoors warmer. This is why, for example, in a residential unit, you get cold air inside your house and warm air outside of your house at the outdoor unit where the fan blows. In the winter when running in "reverse cycle", the heat pump collects energy from outdoors and moves it indoors.
So, to answer your question, "If the water is a few degrees above freezing, where is the heat that is going to be collected and pumped into the cabin at a toasty 75F?" The answer is that even water which is a few degrees above freezing has a ton of energy in it. It gets colder and the cabin gets warmer. Just like in the summer, when the cabin gets cooler -- the outdoors gets warmer. It is the same as when a refrigerator gets cooler inside -- but makes heat by the vents at the bottom.
Having said that, there are practical limits. For example, moisture in the air is a real pain for residential reverse cycle units. In cold weather, especially with some humidity, condensation freezes, insulating things, and preventing the efficient transfer of heat. Most heat pumps have a defrost cycle where they operate for a bit without blowing air indoors to melt this frost off of the outside.
And, it is certainly the case that it is easier to scavenge energy from outdoors to move it indoors when it is warmer outdoors and thereby a greater energy density from which to scavenge. The reverse is true for cooling. As a result, heat pumps work much, much better in moderate climates than they do in, for example, very cold climates.
The fact that heat pumps transfer heat from indoors to outdoors is what accounts for the fact that they are much more efficient, for example, than other types of heaters which create heat from some other form of energy, e.g. burning fossil fuels to convert energy in chemical bonds to heat or resistive coils to convert electrical energy into heat (electrical energy and heat are two different forms of kinetic energy).
As for the mechanism by which heat pumps move kinetic energy from indoors to outdoors, you nailed it. They use a mechanical cycle to compress and evaporate liquid/gas. The compression releases kinetic energy in the form of heat and the evaporation collects it. The refrigerant loop transfers the heat. A fan is used to improve the efficiency of the transfer. And, because work needs to be done to compress the gas, move the fluid, run the fans, etc, the system isn't 100% efficient. In other words, it takes some energy to move the energy, which is why they need to be plugged in (or, rarely, powered by another energy source such as natural gas).
Regardless, yes, these systems do heat the cabin by taking heat from the water to cool the cabin. And, air and water transference are both required. It is just that this transference is facilitated by the mechanical system you describe.
Cheers!
-Greg
