Breastmilk provides all the water your baby needs, even in hot and dry climates. “Breastmilk is 88 percent water. The water content of breastmilk consumed by an exclusively breastfed baby meets the water requirements for infants and provides a considerable margin of safety. Even though a newborn gets little water in the thick yellowish first milk (colostrum), no additional water is necessary because a baby is born with extra water. Milk with higher water content usually “comes in” by the third or fourth day.” “The water content of breastmilk consumed by an exclusively breastfed baby meets the water requirements for infants and provides a considerable margin of safety.”
http://rehydrate.org/breastfeed/faq-exclusive-breastfeeding.htm