Staying warm particularly in the shoulder seasons is something that can also dramatically add to the enjoyment of your time afloat! There are a variety of approaches to this. I will list the options from the smallest and simplest through to the more complex.

We can start off with a couple of simple kerosene lamps. Many boats come equipped with these hung on gimbles attached to a bulkhead in the boat. It is amazing how much heat these can produce if you are relatively free from drafts. Keep in mind that anything with an open flame requires oxygen to burn and may produce both CO2 and CO. Always ensure that there is adequate ventilation.

A second level of simple heat is a propane tent heater. Again, these require ventilation to operate safely.  One downside to these catalytic heaters is that the byproduct of propane is H2O.   You already have a good amount of moisture in your boat, and this may not the most comfortable form of heat!

A step up from these would be some kind of installed cabin heater. There are a variety of oil and propane-based units that are vented to the outside for fumes and will give a good level of comfort in most conditions. By moving away from the tent heater and using a stove with a heat exchanger the moisture and other fumes from the propane is now also vented to the outside resulting in a much dryer heat.

Some boats have very small wood stoves, although these must be constantly tended.

The next step would be to go to a more complex and integrated heating system; either hydronic or fan forced air. Several years ago, I chose to install 50 feet of heater hose around the perimeter of the boat. This was connected into the cooling system for the engine. It also ran through the hot water heater. The heat from the engine would heat the hot water in the tank and then continue around the boat before being cooled by the heat exchanger and raw water system. Connected to this loop are two active heat exchangers that take the heat from the coolant and push it through a radiator with a fan attached. This gives loads of heat that you can direct where you want it. I also have a passive radiator section in the hanging locker to keep things dry in there. One of my active heat exchangers is in the head and the other is in the main cabin. This works extremely well when underway by engine and will work for at least an hour after the motor has been shut down. A further upgrade to this system would be to add an inline diesel furnace to keep things hot when the engine is off.    

There are also forced air heaters that run directly from the diesel fuel in your tank.   The problem with some of these set ups is that they do require a certain amount of electricity and you need to have a way to generate, store and replenish your electrical requirements.