Boiling point in a pressurized system isn't really an issue...the car will go into limp mode due to coolant temps long before the water/coolant will boil. However the corrosion protection provided by coolant is something to definitely consider...that's why its recommended that even if you live in areas that don't freeze to at least use an 80/20 H2O/coolant mix, or use deionized water and put in water wetter, which also has corrosion inhibiting properties.
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