When Can My Baby Start Drinking Water?

The answer to this question is "yes". The answer is also "no"! The full answer is that, like all human beings, babies do need to take in water in order to survive.

But they don't actually need to drink water—in fact, pediatricians advise against giving your baby plain or distilled water until he or she is six months old. Babies get their water needs met by drinking breastmilk or formula, both of which are mostly water. The reason your doctor will tell you not to feed your baby plain water is that it's easy to fill up an infant's stomach; a few little ounces will do the job. Babies should be filling up on nourishment—getting the nutrients they need from each feeding.

Once your baby has been teething, he or she will need fluoride to support thenew teeth, so giving drinking water is a good way to meet that need. But until then, let your baby get water in his or her usual feedings of mother's milk or formula.
If your baby is running a temperature, your pediatrician may advise you to give him or her more liquids. Usually, your doctor will suggest a liquid like Pedialyte, which contains nutrients to restore the balance of your child's electrolytes. Again, don't give your infant water instead of other liquids unless the doctor advises it.

Source : http://www.parenttime.com/babytips/babydrinkingwater.html