Your engine's cooling system is designed to remove heat from the engine to prevent damage. Water alone does not have the properties to not only keep it from overheating and protecting the parts, but to keep the fluid itself from freezing and boiling.
Using the proper mixture of coolant (antifreeze) will ensure proper protection of the cooling system. Be aware that engine overheating may be a cause of other serious mechanical issues and could even affect your transmission and power steering on certain vehicles. You may need to seek the advice of a radiator specialist before servicing your system. A Take 5 Radiator service will remove the fluid from the radiator and engine block replacing it with the proper mixture, level, and type of fluid needed for your vehicle in only a few minutes.