"Too much water can cause water intoxication in babies. Each time a baby pees, he or she loses not only water but sodium and other electrolytes. But unlike with adults, who tend to get too much sodium in our diets, babies usually get just the electrolytes they need from breast milk or formula. Too much water and they lose too much sodium. The sodium levels in their blood can plummet and cause irritability, brain swelling, unresponsiveness and seizures. (Note: Water intoxication also involves other factors, but that’s more detail than I can go into here.)
The risk of water intoxication is especially high if the baby is losing both water and electrolytes from diarrhea. Fluid losses should be replaced with breast milk, formula or perhaps a rehydration solution. Don’t give plain water for rehydration.
The risk is also high if formula is diluted with too much water in a misguided effort to save a little money."What about dehydration?
"If you are concerned that your baby is getting dehydrated, contact your health care provider. We expect babies to pee at least once in the first 24 hours of life, at least twice in the second 24 hours, and at least three times every day thereafter."
Should you boil water given to young babies?
"Boiling is the most certain way of killing all of these organisms. Even though the risk of any infection is probably far below 1 percent, I agree with the American Academy of Pediatrics in recommending that sterile water be used for young babies, even when used to prepare infant formula. In practice, this usually means boiling the water for one minute, at least for the first couple of months."