Below 45° F water temp, it's a lot less effective. It's the same system as in your house(s) -- except the medium is 'to water' instead 'to air'. The transfer may more effective than an split system trying to work with "air" vs. "water" -- but still, if you're in cold water....

I know people that have tried to live aboard with the reverse cycle that ended-up with ceramic heaters. The water temp in the Chesapeake gets down to 34° (or less) in a cold winter. It's a bit warmer a couple of feet under the surface, but not much in February ;^)))

The our reverse cycle AC units are okay in the summer up in your neck of the woods, Tom. The problems:

i) the discharge vents for AC are usually high for a variety of reasons: Effectiveness, comfort, and where the larger ducts can usually get installed; and,

ii) the runs on 110 v, so we'd have to run the generator or be hooked up to the shore power; and,

iii) the AC units are noisier (not including the generator) because they really work (at least all that I've had) with a constant fan and then their compressor(s) then kick-in and -out. If you're in cold water, the compressor is on almost all the time if you're going for normal temps inside the boat.