I'm going to disagree. Water, alone, is about a B+, maybe an A-. If you've ever been working out really intensely, to the point where you feel nauseous and could drink a liter of water and still be desperately thirsty, then you'll understand that you also need to get electrolytes, things like sodium, potassium, and magnesium. The mineral content in plain water is too low for that; a sugar-free (preferably unsweetened) sports drink is going to be better for you than water alone.