I installed two Mermaid units in my boat shortly after I built it. A 12kbtu in pilothouse, a 6k shared in fwd cabins. They have worked well for 7yrs/1800hrs travel/11,000miles. They have reverse cycle heat, and with water temp in the 40's they can freeze up. Usually the freeze up occurs right after startup, so if it makes it through the first hour, all ok. A short run in ac mode melts the ice block, still a bit of PITA. Water temps 50 up, no problems with freeze up at all. Thermostats are std house type, so that is a plus.
No repairs have been needed, except one thermostat pooped. Quiet, too. The 6k is under the bunk I sleep in. It does not wake me up.
Boat also has a bus heater connected to main engine coolant circuit, that takes care of heat when under way.
I also keep two 1500 watt cube heaters on the boat. If it is real cold outside (20F) I run one in each cabin. Toasty. The 6k is undersized for those temps.
I've done a lot of winter running, and the above combo really works.