Your truck's cooling system doesn't operate on coolant alone, and it can't operate on just water, either. In fact, your truck's cooling system actually requires a mixture of coolant and water to provide just the right level of protection for your engine. If you skew the manufacturer's suggested coolant-to-water mixture by adding only water or adding only coolant, you could risk damage to your truck's engine.
The balance between coolant boil-over temperature and freeze-up temperature actually hinges on the proper mixture of coolant and water. For instance, a mixture that consists of 60 percent water and 40 percent coolant can provide boil-over protection all the way up to 259 degrees Fahrenheit (126 degrees Celsius) and freeze-up protection all the way down to minus 10 degrees Fahrenheit (minus 23 degrees Celsius). If you were to change that mixture to 30 percent water and 70 percent coolant, you would have boil-over protection up to 270 degrees Fahrenheit (132 degrees Celsius) and freeze-up protection down to minus 62 degrees Fahrenheit (minus 52 degrees Celsius) [source: PEAK Performance Products].
As a general rule, you'll find that a 50/50 mixture of coolant and water is sufficient for most driving conditions. However, to ensure that you're using the proper mixture, it's a good idea to take a look at your owner's manual.