Will you be living aboard? Do you want to just heat at the dock or will you need it cruising? The answers will help narrow down your choices.
Our boat came with 2 reverse cycle AC heaters(Mermaid). we use the AC at the dock in the summer because we cannot always be facing into the wind. We use the heating component in the shoulder season at the dock.
For winter and heat at anchor we installed a 12v Hurricane H2 diesel furnace. It is a hydronic system and it actually helps keep the boat drier. The furnace is in the cockpit lazerette with exhaust out the transom so so worries about a flame adding moisture to the boat. It has a water jacket with a glycol mixture that gets pumped throughout the boat to small fan/radiator units. We have 3 zones with programmable thermostats. Aft cabin, Salon/galley and forward cabin. It was 9° last night and we were toasty warm. This extends our season in Maine from April to December now that we can comfortably anchor out and stay warm below.
Our fuel tankage is 210 gallons which usually gets us through the winter.
This heater also has the capability for on demand hot water and engine pre-heat. Having the heater in the engine area also keeps it from freezing so I never have to winterize. I do run the engine at least monthly to keep her well oiled.
See my projects link below for more info.