I think Seaward made two versions, SS and Aluminum.

I have a galvanic isolator, and use the non-toxic anti-freeze.

Scout wrote:
How many here have a hot water heater that works with the engine's
coolant system as a heat source?

It is a very thin line separating that toxic material from your drinking
water supply. The closed engine coolant loop may reach 15 psi when hot,
and could easily outmatch the force in the fresh water system,
especially if you run dry or turn off your on-demand pump for any
reason. A pin hole or a crack in the hw heat exchanger could have deadly
results.

In my work as a boiler tech, we were never permitted to use toxic
anti-freeze in boilers that produced domestic hot water (summer-winter
boiler packages). I've replaced tens of those coils due to leakage!

Just a thought, but Bart's recent comment about galvanic isolation got
me thinking.

Maybe I'll get that RedDot heater that Ole Thom has mentioned, remove
the anti-freeze loop from my fresh water supply, and use those lines to
supply aux heat in the cabin.

Scout