We have an after market fresh water cooled Yanmar 2GM. What happens is the fresh side is circulated with the existing pump
. This pumps
the fresh through the engine and to the hot water heater, then back through the heat exchanger, and finaly back through the engine block. An additional pump
, belt, and pulley are mounted for sea water. This pump
brings sea water in, and is plumbed directly back to the heat exchanger and then to the exhaust
mixing elbow. In my system the water heater will keep water hot all night. If I motor for about an hour in the evening to top up my batteries, that will be enough running time to have hot water the next morning. My water is really hot too, I have to use the mixer so I don't scald myself. So, you now have hot water, and as a bonus you no longer are introducing salt water to the engines cooling passes.
Our set up is small, but I would think the principals apply to larger systems too.