My $0.02....
I would not want to drink water that had been directly exposed to a refrigeration heat exchanger. The materials used in those systems are NOT designed for potable water... so do we take a chance on the lead solder, etc used to construct them? Also would not want to taste the refigerant oil in my water should the heat exchanger leak... the higher relative pressure in the system would discharge the refrigerant & oil into the lower pressure coolant loop and your water tank.
So why not build a closed system? With good access to your water tank, take a small transmission cooler with a dense finned area and plumb it to the heat exchanger. This would require adding an inlet/outlet to your water tank, but it's still not another thru-hull.
It's entirely possible that even with an empty water tank the finned unit would provide enough of a temperature drop to support the system. They also make finned tubing sections that could be used in the run from the heat exchanger to the tank to help dissapate the heat.
For that matter - why not simply install one of those keel coolers into the water tank?? Assuming you have access near the bottom of the tank, one hole cut would be all that's needed. Just have to be careful about the materials they are made from - be sure they don't include some type of anti-fouling chemical or other material that would cause a health hazard.
For me a closed system would give me peace of mind. I've seen too much "interesting stuff" in refrigeration condensers to want to drink from them.
