So no one has done this ? Well I have.
No room for a decent calorifier or instantaneous heater, aircooled engine, no room for a heating coil from fire.
So we have a 12v 10 litre calorifier.
It is 300w at 12v or 1000 at 240 but we have no mains on the boat.
We have 3 110 batteries including the start and once boating and the advec has stopped , put on the hot water.
The buffer of the batteries means the charge rate is about 25 amps.
We turn the heater off 30 mins before tying up
The alternator ( 127 60 amp) has sustained this abuse for 4 years to date.
I rewired to match the load with seperate cables.
We never heat water unless under way .
It works ok, not ideal but pragmatic.