As OC Diver said, as long as you're moving enough water through the system to keep the temperature drop across the coil down, the system will work down to a couple degrees above the freezing point of the water (so figure down to 35* in fresh water, 30* in salt). Below that, you'll start to freeze water in the heat exchanger, so it's time for a different heating solution.
My biggest issue with reverse cycle units being the only heat source (which is my current situation) is that it pretty much leaves you marina hopping unless you don't mind running a generator 24/7. And the generator also becomes mission critical, as no 120v power = no heat.