From the back of the Diet Coke Box:
"It's true. Research shows that all beverages contribute to proper hydration. That means whether it's your first can of the day or your afternoon pick-me-up, Diet Coke helps you stay hydrated all day long. So stick with the Diet Coke taste you love. Your body will thank you for it."
My favorite part is the syllogism started in the second line. 1) All beverages hydrate you; 2) Diet Coke is a beverage; 3) Therefore, Diet Coke hydrates you.
I understand the motive behind adding this statement to the box, what with the bottled water craze. But when your strongest selling point is that your product shares a trait with every other conceivable alternative to your product, maybe you should just focus on how delicious your soda is.