I too was wondering the answer to this question so i looked it up on babycenter.com
Starting at about 6 months a baby can have water ...
Hold off on giving your baby water until he's about 6 months old, and limit it to about an ounce or two at a time -- ideally after meals. You don't want your baby to feel so full that he refuses to nurse or take his bottle. If your baby seems interested in taking occasional sips of water before 6 months of age, it's not going to hurt him. In fact, a little bit of water can help keep your baby hydrated in hot weather. But talk to his doctor if you're offering it before 6 months. Giving a baby younger than 6 months old too much water can interfere with his body's ability to absorb the nutrients in breast milk or formula. In rare cases, babies who drink too much water can develop a condition known as "water intoxication," which can cause seizures and even coma.
"Babies don't really need water, or any liquid other than breast milk or formula, until after their first birthday," says Rachel Brandeis, a registered dietitian in Atlanta. After age 1, water and milk are Brandeis's baby beverages of choice. "You want to be sure juices don't displace milk in a baby's diet," she says. Once your baby celebrates his first birthday, you can let him drink as much water as he wants.
When you introduce water to your baby, offer it lukewarm, especially if you breastfeed or usually give your baby formula that's warmed up.