Jay is right, and yes, Distilled water is the way to go, Tap/Spring/Well water had lots of minerals/salt, ect that will promote corosion inside your cooling system. The only reason we change coolant is because the anti-corsion elements of the coolant breakdown after time, however, the ant-freeze elements will not breakdown. Make sure you follow that 50/50 ratio too. All coolant is just as bad as all water.
For extreme hot/cold protecton, I think a 70/30 is suggested, but check the coolant label to be sure.