1.Never give extra water to infants younger than 6 months because it can cause a seizure(water intoxication caused by electrolytes imbalance).
2.It's not needed. (Reason: breast milk contains 88% water)
3.If your baby gets adequate breastmilk, additional fluids are not necessary and may decrease your baby's interest and ability to breastfeed.
http://www.livestrong.com/article/206777-should-i-give-my-baby-water-to-drink/