It isn't necessary to give your newborn water.
Most babies get enough fluids from breast milk or infant formula.
Even when a mother's milk supply is just coming in during the first few days after delivery, the baby is getting colostrum ,which is enough to keep the baby well hydrated. Because a newborn's kidneys are immature, she has a difficult time concentrating her urine, and if given too much plain water, may have trouble excreting it. This could lead to dangerous changes in the concentrations of sodium and potassium in her bloodstream. For these reasons, infants should not be given extra fluids unless their pediatrician recommends it. As with any source of medical or health care information, you should as a preliminary matter satisfy yourself that the credentials and experience of the source are suitable.