It must always be pure distilled. Any additional trace elements will build up in your cooling system.

Running 100% water isn't dangerous most of the time, but this drastically lowers the boiling point of your cooling system. Which means a greater chance of draining your system through the puke valve in hot traffic.

Whatever your backyard mechanic may tell you 50/50 or 70/30 water/coolant is the best combination to maintain your car's engine. In fact, some newer vehicles come with a waterless cooling system that's more effective than the common 50/50 kind.