the usual is a loop from the engine through the water heater
so that the water heater is warmed. this is a heat exchanger
so no engine water makes it to the water tank. In my system, the engine is glycol cooled. hot glycol passes through the heat exchanger
in the water heater before it goes to the sea water heat exchanger for massive cooling
. There is also a circuit of hot glycol passing through a reverse cycle heating
system for the air conditioners.