Don’t forget that you won’t dry out the cabin and boat unless you let a lot of the warm, wet air out. The reason warm air dries is that when air is warm it can carry more water vapour than when it is cooler, therefore if warm, wet air cools it will reach a temperature where it becomes saturated with water vapour so any further reduction in temperature will lead to condensation of liquid water on any surface that is colder than the air.
Therefore to dry out a boat, or any other space, you need to heat the air so that it can evaporate any liquid water present and carry it as moisture vapour then let it out so it takes the water away.
As an example: air at 20°C can carry up to about 17 g of water per cubic meter of air without condensing out as liquid (it is said to be saturated at 100% humidity at 20°C). If this air is then cooled to 10°C it can only carry about 9g of water per cubic meter of air at this temperature so the difference - 8g in this case - will condense out as liquid water on any surface that is below 10°C. If you just heat the air in the boat but don’t let it vent out it will remain warm and damp inside but it will not dry out. If you want to dry it out you need to allow the warm air to leave the cabin after it’s been there a while to allow it to pick up water vapour.
Therefore if you just want to stay warm the heater needs to be in the boat and just heat the air and retain it within the cabin but if you want to dry the boat you have to heat the air and let the warm air escape after it has taken up water vapour. Clearly the former is more fuel efficient than the latter as you have to let the warm air escape.
I've attached a little graph that allows you to see how the water vapour changes with temperature for saturated air.
As you can see you need to put a lot of warm air through a boat cabin to remove any significant amount of water. A large fan heater left running for a few hours with the hatches open is probably the best way to dry out a cabin.
Hope this helps
Mike C - Tarka 26M