I have real first hand experience with a different brand of hydronic heater and it does work very very well. We had a 46,000 BTU unit in a 46 foot boat, with three zone heaters, plus passive radiators in the heads and hanging lockers. The current draw was 5.6 amps all running, including the fans in the radiators. We used a fire box from one manufactuer and radiators/controls from other manufacturers.
The espar will work the same as the unit I had (Hopefully as well). There was no diesel smell. The fuel was compressed by a fuel pump and air was injected with an air compressor. There was a forced draft exhaust and a circulation pump for the water. The water was maintained at 180 degrees (too hot for showers). Since we were running the main engine or genset daily for refigeration, we did not hook up a heat exchanger to the water heater or engine block. In our unit the liquid circulated was an antifreeze solution, that is why a heat exchanger would be necessary.
If I was to do this, I would take advantage of the heating function with several radiators in the boat. Our radiators were about 8" x 10" with multiple finned coils and 12 volt computer fans. We used a Robertson thermostat.
I am sure that you can run the output directly to a shower system--but I would prefer to have a small tank for storage, and equalization of flow etc. Frankly this is a fairly expensive "solution" to water heating--but if you want to heat the entire boat, it would make a lot of sense. Our system cost about $5,000 in 1992--and that was with discounts and doing the labor. (ours was ProHeat burner, and it is another boat now, still ticking away 15 years later, with no service problems). Our system was in a large engine room--the fire box was about 1' x 1' x 2', with a reserve tank of about a gallon about 3' above the system. You can get really serious problems, if you run the system dry--and that is why we had a heat exchanger system. The Espar may be able to be run dry, or have safety cut offs. We ran the liquid thru 3/4" hot water heater hoses, all thru the boat, and the passive radiators were 3/4" copper pipe along the floor--in some places coils back and forth behind cabints.
I would look at the clearance necessary. Our unit was completely housed in a diamond plate aluminum cover, and we had 6" clearance all around, with lagging on the flexiable exhaust hose, which went thru the hull. The air intake came in around the exhaust, so it also helped to cook the exhaust. The hyronic systems are the best way to heat a boat in Alaska.