don't run pure water.. water boils at 212 degrees.. And if that happens in your car, your screwed.
This is the whole reason why antifreeze was made. It provides protection in cold weather, by lowering the freezing point, and provides protection in the heat, by raising the boiling point of the fluid. Then add in a radiator cap to pressurize the system, the pressure also adds to the boiling point. So a 50/50 mix is the least people should run. I can't remember off hand, but there is a certain mix(AF/water) that provides the best (highest) boiling point. I want to say its somewhere around 30antifreeze/70 water.. Don't quote me on it, but its more on the water side.