Coolant is a mixture of water and chemicals designed to keep your engine at the correct temperature and prevent corrosion. Water is just...well, water. It will evaporate quickly, especially in hot weather, and it will not do anything to protect your engine from rust or overheating. So, if you are going to use something other than coolant in your radiator, stick with good old H2O. But at last, coolant is better than any other substance, whether it is water also.