Contrast all of that with a relatively cheap solid fuel fire. They come in a variety of styles, some with a back boiler to heat radiators or the domestic hot water (although their strength is with their space heating, rather than as water heating systems). They provide a cheery winter glow with various types of coal bought anywhere round the canal system (or use free wood from fallen trees). A solid fuel fire draws in the cooler, water vapour laden, air at the bottom to fuel the burning process. The water content drawn in, or produced in burning, is then expelled from the fire box interior, up the chimney - providing a warm AND dry boat. It isn't a matter of whether or not water vapour is produced - The real issue is whether or not the system gets rid of the water vapour that IS produced. A boiler located under the back deck is going to do little to expel water vapour (condensation) found in the saloon or galley, whereas solid fuel fires (and drip-fed diesel systems) located in the saloon DO get rid of that condensation. Positioning of the fire can also be important - for the boat fitter it is very convenient to put the fire right beside the front door - but heat rises straight out of the boat's front door every time its opened. We have our fire as far towards the back in the lounge as the fit-out will allow (see the marginal photo) - much better use of the heat.  Sometimes the old tried and tested solutions work better than some modern alternatives. 

However, there are drawbacks to having only a solid fuel fire.  There are autumnal evenings when its chilly enough to need heating, just to bring the temperature up a bit then, when we light a fire, an hour later everyone is needing to strip off - and the fire won't go out until 9pm the next day.   So a secondary system which can be turned on and off, at will, is very useful - or you could just burn wood on your fire (which will go out quicker if left alone).

For Domestic water heating, a calorifier can be put to good use (looks like a domestic hot water tank - but designed to withstand the higher water pressure found on a boat). A Calorifier is actually an efficient heat exchanger. The main part  of the calorifier tank contains the water to be heated, supplied from the boat's cold freshwater supply.  Hot water from, primarily, the boat engine cooling system then passes through copper piping which, in the form of a coil, passes through the cold freshwater within the calorifier tank.  Heat passes from the hot engine coolant within the coil, through the metal of the copper pipe coil, and heats the main body of cold fresh water within the calorifier tank.  That heated freshwater is then drawn on by the boat's hot water taps, as and when required.  So, whilst cruising, we have hot tap water for free - has be good.  Additional coils are usually present to allow additional sources of hot water to do the same job (egs hot water from sources such as the gas/diesel central heating boiler or even from the back boiler of a solid fuel fire).