Good Housekeeping did some thorough research on zero-calorie and enhanced waters. They found plenty of sugars, artificial colors, preservatives, false promises and hype, and very few products that are really good for you.
Their pick: Ayala's Herbal Water: "It contains nothing artificial, not even inflated promises. The herbal mix-ins will likely add some antioxidants to your diet and, if you don't enjoy the taste of plain water, help you drink more fluid".