As an FYI, the products found in stores are flash-pasteurized to kill bacteria so they can sit on the shelf for a bit. Because of this process, some of the nutrients found in fresh coconut water may be destroyed. If you don't have access to fresh young coconuts (not likely here in the upper Midwest), some health food stores, like Karen's in Lincoln Park do sell fresh, unpasteurized coconut water (approx $7 for 16 oz). That price is high, but in theory you would need less of the fresh stuff to get the same effect ...
If you can't find the fresh stuff, the grocery store brands are still a good, safe bet. And if you're not buying the coconuts and making the juice yourself, you should stick to the pasteurized juice if you're giving it to babies or are pregnant or nursing.
Always consult your doctor if you or your child has a serious illness and before giving a baby a new food or drink.
* Unpasteurized juice may contain harmful bacteria.