hI designed and built my own hydronic circulating hot water system. We used a on the road truck unit (I am blocking on the name of the Canadian firebox/pump currently). it was 45,000 BTU, fuel pump, air compressor to help atomize the diesel & blower for the forced draft exhaust. (Also brought in external air for combustion chamber). We had 3 radiators, one in the main saloon, one in the forward stateroom and one into aft stateroom. These were copper tubing finned type with a computer fan on a thermostat. The fans came on at a certain temperature you would set. The circulating water was 150* and the unit would cycle on and off in the limits of 145 to 155 degrees. We didn't include the water heater, since we ran the genes every day for making water, charging batteries, etc--or it ran off the main engine. There are copper coils under the floor in the heats, and in the wet hanging locker. Each head had a towel rack with coils behind it. It worked very well in AK.
Needless to say this is a unit far greater in capacity than you would need. I probably would put one fan powered radiator in the Foreward bunk and one in the main saloon.
We also ran a 25,000 BTU "Red Dot" radiator off the main engine--that was in series with the water heater. It would heat the main saloon, and if the hatch was open, the cockpit.