Hyponatremia, also called water intoxication, can be fatal. There are adult marathon runners who have collapsed and died after drinking too much water, and with their low body weight, it's even easier to throw off a baby's electrolyte balance. Check out this link: http://www.stlouischildrens.org/arti...ion-in-infants
You might want to distribute the AAP policy statement on breastfeeding (http://pediatrics.aappublications.or...115/2/496.full) to your family and doc, with this bit circled: "During the first 6 months of age, even in hot climates, water and juice are unnecessary for breastfed infants and may introduce contaminants or allergens."