When my boys were infants, I remember reading all the books, and recall that for those first few months of life, breastmilk was all they needed to eat.
I also remember a family member who asked if I should be giving my baby water. I glared at her, and then responded that my breastmilk was enough. Even though she also asked if my breastmilk was enough, I let her remarks slide.
Now obviously after solids are introduced, babies can learn to drink water from a sippy cup – but never before because it could be dangerous.