I think these systems are primarily in sailboats, to have hot water for
dishes and showers while at anchorage. Just like refrigerators that run
off of an engine mounted compressor.

Bob Dimond

Most marine water heaters can work with either electricity or the circulation
of engine coolant through a coil. Has nothing to do with sail vs power. Heating
the water
with engine coolant means you can have hot water underway without running a
generator.