Chris brings up an interesting topic and the initial link I offered includes several links to studies, some of which seem to contradict each other, on that topic. Among them (even more if you check out that page, I don’t want to just offer every link on that page):
WATER AS GOOD AS BRAND-NAME FLUIDS FOR MAINTAINING HYDRATION WHEN EXERCISING IN THE HEAT
WATER A GOOD FLUID FOR MODERATE ENDURANCE TRAINING
SODIUM-CONTAINING BEVERAGES SHOULD BE USED WITH HIGH SWEATING
Of note with so many people paranoid about experiencing even mild dehydration would be the following:
MILD HYPOHYDRATION DOES NOT AFFECT PHYSIOLOGICAL MEASURES IN COMFORTABLE ENVIRONMENTS
EXERCISE OUTPUT NOT AFFECTED BY BODY MASS CHANGES DUE TO HYDRATION