Actually using a copper coil in the water tank would be the safe way to use the fresh water tank
as a heat sink. As a previous poster pointed out you can get deposits in the fresh water tank
, which would be annoying at the least. Installing a copper coil in the bottom of the tank and making the refrigeration
look closed would solve that.
I would add a small water hammer arrestor to act as a small thermal expansion tank and maybe a watts 630 pressure relief set at 40 psig too allow for thermal expansion should things go other then expected. You also would need a fill valve to fill the system and bleed the air out.
Me I would just run a keel cooler
/condenser and get rid of the extra power and cost of a water pump and the fiddly bits. If you wanted to get fancy, put it in the fresh water tank, then no holes in the hull