We buy premixed coolant by the semi-trailer load for industrial engines: it's glycol and deionized water, mostly because at some locations the water is of poor quality, full of iron, etc., but also because we can be certain the corrosion inhibitors are there and the freeze point is correct for the location. We use enough glycol to get appropriate freeze protection, but no more, as the cooling capacity of the systems is reduced as the glycol concentration increases (but the boiling point in a pressurized system increases, I know; these aren't pressurized as cars' are).
I opened my Benz up after 20+ years of somewhat indifferent care by the PO and found... nothing wrong in the cooling system. It had probably had the same green stuff in it for 5 or more years. Our tap water is fairly soft and doesn't cause problems. I have a car I've owned since 1974 and no problems ever with the cooling system, with just general good maintenance, nothing elaborate. If your tap water does not ruin the coffee maker, sinks, toilets, or water heater, you probably won't have any problem with it as long as you use the right coolant and change it every so often; if it does, don't use it in your cooling system.
Don't spend all your time worrying about your car; spend most of your time enjoying it.