Nothing like a shower...Engine water heat exchanger. I think Jeep makes one like this. That is where I got the idea. I would assume this would work better with a sealer engine cooling system. Mine is raw water (salt in my case) and has been in use with no issues for over 12 years. I'm not looking to have a hot shower, but just to temper the tank water. Be very careful, the water can start off scolding hot!
I would suggest adding a mixing valve to add cold water and adding a more even temp. and longer run time.
I ran the hot water to a hand held sink type spray head I keep coiled up in the Port Lazarette. Can also be used to wash down the cockpit. Or two birds with one stone.
What this plan does not point out is a 2 1/2" to 3/4" reducer on both ends.
And that would be my rebuilt Graymarine Sea Scoot 91 engine in that pic. Did that myself. Love to keep the original if possible.