With small ducts needing to carry hot air to the bow from the stern(close to the exhaust vent), that could take some tricky balancing of fans what with air friction in the long ducts and heat loss along the way. Compound that with a boat which is a pretty poor house to begin with it's insulating values alone not to mention the need for ventilation due to the small areas involved.

Hydronic sounds a little easier to get the needed energy the distance and distribution. That's a lot of stuff going on that needs extra cost upfront and during operation to offset the loses. The bigger and more complex the boat, the more sense it would make as they can generally supply the extra power.

In some ways, the single heat source has an advantage. It was (and still is) used by fishing boats that work in harsh cold wet conditions. The first were solid fuel followed by oil drip burners. Still the same concept, a hot radiating heat that would warm the central area(the only area in many old fishing boats) and anything/one in the radiation.

I would think that's why these radiating heaters(Like Reflex?) still are for sale for fishing and pleasure boats. It's likely a better solution for some boats and how they are used. Not many sailboats have any potential installation areas left over after all the bunks and what not are installed.