One of the marine water heater operating manuals that I read on line said
that if antifreeze comes out of a faucet, the heat exchanger should be
replaced! I also read somewhere that non-toxic antifreeze (polypropylene
glycol) was not recommended for some engines.

Pressures of water and engine coolant pumps cannot be known for every boat,
so I would think there is a good chance the coolant side is at a higher
pressure than the domestic water, esp if the domestic water pump is switched
off.

So, I think you have a good point. I suppose S/S or Cupro-Nickel coil would
help, but with stray currents, anything can happen and probably will.

GBM

I have a generic concern about water heaters that use engine coolant as
the heat source: a pinhole leak in the heat-exchanger tubing inside
the water heater would permit coolant to mix into the hot water, and
you might not know it until the leak became severe. Ethyl glycol is not
my favorite intoxicant!

Anyone aware of any instances of this (presumably, there would be a
lawsuit)? Are my fears unfounded?

Capt Ron