Generally, breastfed and formula fed babies should not be offered water or other liquids until they are about 6 months old. Breast milk and formula are almost 90% water, so it is not necessary to add additional liquids to their diet. Offering additional water can potentially be dangerous to infants. It can lead to health risks such as jaundice or low weight gain, and can additionally decrease mother's milk supply, as baby is demanding less from the breast.
Once your baby is in a solid food regimen, you way want to offer a few sips of water, formula, or expressed breast milk in a sippy cup to keep them from getting constipated.