On our last boat (Catalina 36) we had a WABSCO Forced Air Diesel
Heater... It worked great for the San Francisco
winters (and Summers :-) ) and burned something like a cup of diesel per hour.
On our present boat we have Marine Air AC Units... They wroked good for heat in San Francisco
, but hte water temp stayed in the 50's.
Were these AC units really showed there colors was when we started cruising in the tropics and spent a rainy season in Central America
. They reduced the humidity to the point the inside of the boat had very little mold
Although we have three units on the boat, we never usually run more than two at a time, which keeps the boat comfortably at 75 degrees without shade covers on the cabin
Talk with a marine AC guy about how many you need and were to install them. Outside drilling the thru hulls, there is nothing hard about instlling those either.