As with any strenuous activity, you have to make sure to drink enough water and eat enough food, even when you don't feel like it.
It's not unusual to face 120-degree Fahrenheit (about 49-degree Celsius) heat on the Colorado River. And there's a saying about working in the heat: Once you're thirsty, it's too late. Thirst, they say, indicates you have already started to get dehydrated. The NPS recommends downing at least a gallon per day. Water is great, but you can also choose fruit juice or a sports drink like Gatorade. Of course, this is a vacation, and some rafters will want to partake in alcohol while camping. But alcohol, as well as caffeine, are diuretics and contribute to dehydration. So, if you drink alcohol and caffeine, do so moderately, and make sure you're getting plenty of hydrating fluids.
Of course, it's good to bring your own water, but if you run out, you'll want to know about the available water on the trip. Unfortunately, water from the Colorado River itself isn't safe to drink, and neither is the water from the side streams and springs. To disinfect the river water to make it potable, the NPS lays out some options.
First, any cloudy water you gather should be left alone for a few hours to settle any sediment at the bottom. Then, pour the clear water into another container and run it through a micron filter. To ensure disinfection, finish by adding two drops of bleach or five drops of tincture of iodine to each gallon of water. Let it sit for another 30 minutes (longer for extremely cold water). If you have enough material for a fire, another option is to boil the cleared water for a minute.