I would say the tap water is
I would say the tap water is sufficient to get your car to the service facility, BMW will replenish whatever coolant is lost during the repair with new coolant (not reused) as for the minerals in standard tap water, replacing your coolant every 3-4 yrs will prevent corrosion even if tap is used rather than distilled. Anybody who believes that bmw's rating of lifetime coolant is a fool. BMW just uses a lifetime service interval on their vehicles to bolster their low maintenance cost claims. 14k oil changes, lifetime transmission fluid, Haha! Try getting 200k reliable miles from that car, not happening!