He suggested that I pick up a car/truck heater and plumb it
into the hot water heater system. It would provide heat while we're under
way and allow us to go below and warm up once in a while.


That's what small power boats have.

If you can find a stove you can live with, there is an old trick for heating
when at the dock. Put stove on low heat, invert a traditional terra cotta
ceramic plant pot and put it over the burner. It will heat up and radiate
nicely.

Henry.