My understanding of heat pump
/ac type units that lack additional resistive heating elements is the most efficent units can only heat or cool approximately 30 degrees from ambient.
So on a 100 degree day, ac is only going to get you down to about 70 degrees. Conversely, if its 20 outside, about 50 is all the "heat" you can expect from a heat pump
unit. You are gonna need some alternate means of heating your boat when the temps get below about 35 degrees.
Home heat pump
units have a "second stage" to take over when the heat pump
alone is not enough, but marine units lack this feature. The second stage is usually electric resistive elements.