They don't need water, they get plenty from breastmilk or formula. I didn't start giving my son water until he started on solid foods at 3 months to help with constipation and I didn't give him much at first. Now that he's about 8 months I give him a sippy cup of water all the time and even still I breastfeed him so he probably doesn't really need it.
There is such a thing as giving an infant too much water actually.
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