Babies under 6 months old have very small stomachs and since the baby has a small stomach if you give the newborn baby water it will likely fill up their stomach too much which will decrease the babies appetite and then they won't want to take their formula to feed on.
When the baby won't take the formula since the baby is filled up with water instead then the baby can become sick because it will become malnourished.
After 6 months of age then the baby could be given water but before then you should only give the newborn formula or breast milk so the baby will be nourished and healthy.