I have one of those units...... life is just a better place when you have instant hot water.
although I havent used it on the boat, it will work anywhere.
I carry it in my camp trailer and anywhere you have a water source, you have a shower.
I set it up and use an RV water pump to suck water out of the creek.... push it thru the unit and I have instant, on demand hot water.
the temp of the water is adjusted by the amount of flame you have burning so you dont need a mixing valve.... its a great unit.