Coconut water is supposedly quite hydrating as well and, unlike Gatorade, natural. It's a shame they did not test it but the reasoning applied to milk should carry over:

"Why is milk so efficient at rehydration? “Normally when you drink, it signals the kidneys to get rid of the extra water by producing more urine,” Dr. Maughan said. “However, when beverages contain nutrients and electrolytes like sodium and potassium, as milk does, the stomach empties more slowly with a less dramatic effect on the kidneys.”"