Plain water is the least expensive and most readily available. Wood says to avoid concentrated beverages, as they may actually contribute to dehydration. Also, avoid alcohol and any kind of beverage with caffeine, because they act as diuretics, and increase the loss of fluid from the body. Light-colored urine in normal amounts indicates normal hydration. The average amount of urine expelled per day varies among individuals, but it is usually about one and a half to two and a half quarts.
Wood says heat-related problems are preventable and fairly easy to avoid.
At the very least, stay in a cool place and drink lots of liquids. Use a little common sense, and you won't have a problem when mother nature turns up the heat.