We use an Eberspaecher (Espar) hydronic heating
system on our boat, which we use all year round in a climate which includes a certain amount of below freezing temps. It burns diesel and works well.
On your boat I would say a Webasto or Eberspaecher forced-air diesel-burning heater would be the best thing. The bulkhead mounted heaters will work, but they don't distribute the heat nearly as well. The forced air system distributes heat and circulates air and will help a lot keeping down condensation
, which will be your next big problem right after you solve the one about not freezing to death.
Disadvantage compared to a bulkhead heater is you will need a certain amount of electrical
power. You say you don't have AC -- do you run the main engine
to recharge your batteries? Running a forced air heater 24/7 will demand a fair amount of power. You might need a suitcase generator
, which is already adding more and more complication.