"poisonous antifreeze can get in to the domestic water supply. Is this something that anybody worries about?"
Not much , its taste is easy to spot , although animals have been harmed by drinking discarded Anti Freez.
You will want to put a bypass thermostat between the coolant return and outlet.
This stops thermal shock of the cool water returning to the engine.
The stock thermostat keeps the water at operating temp, but only a thermostat can keep the coolant return proper temperature..
It is usually 20deg F below the engine operating temp.
This hassle occurs as a proper keel cooler is properly sized for full power operation in tropical waters, uncommon for cruisers .
So there is huge over cooling , with out the bypass thermostat.
Only 1/2 a boat buck or so, and a foot of hose.