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WS, I'm told the Espar/Esbacher marine heaters were all spun off their diesel BUS heaters, so you're not far off the mark. I'd expect a marine model to have some different accomodations for venting, and perhaps to use some different materials. For instance, copper and aluminum are just fine for all sorts of hoe and auto use, but mix them with fluids and salt air...not so good in marine use.
When you say "furnace" I think hot air, like the bus heaters, but when you say "hydronic" that's what is (to me) usually called hot water baseboard heating, run out through metal pipes and fins. The latter might be an issue in marine use.
I will say that is there's an Espar (etc) hot air heater in your boat, it kinda spoils you. Nice and toasty DRY.