There seems to be some misunderstanding about this, so I want to be thorough about when and why babies need water. For many years, parents gave all babies water from a very young age. As exclusive breastfeeding became the recommendation there were still many well-meaning grandparents out there trying to give breastfed babies sips of water. ("They must be thirsty!") But breastmilk has all the water a baby needs. In fact, it's 90% water. Extra water will lead to baby drinking less milk, which undermines the benefits.
This has led to...
Breastfeeding advocates have been so adamant that newborns don't need extra water that some mothers think that all breastfed babies under 1 don't need extra water. This is false. Key word: Exclusive. Exclusive breastfeeding means baby's not getting any nutrition besides breastmilk. Once a baby starts eating solid foods they're no longer exclusively breastfed and they also need additional water. The American Academy of Pediatrics states, "with the introduction of solid foods, water can be added to your baby's diet. Also, a small amount of water may be needed in very hot weather." To put it scientifically, "Infants need additional water when solids are started because of their osmolar load." - Breastfeeding and Human Lactation (2016), p. 700.
In real words, please?
OK, so to simplify it means that there's a certain amount of nutrients a human needs, and the water in our diet needs to be in proper balance. Too much water and not enough nutrients are available. Too little water and the first sign is usually constipation. The first thing your doctor will tell you to try in the event of constipation in an otherwise healthy baby eating solids is usually just some sips of water to help everything move through normally.
Don't believe me?
Maybe someone you know says they never fed their child extra water and they didn't get constipated. Just look at the ingredients on every jar and pouch of baby food and see that water is in them. It's true that if a child eats lots of fruit, soup, and other foods containing water they might not need it from a cup, but the point is that they're still getting it in the food. It's when we add things like dry cereal, pretzels, and crackers that a lack of water will be more obvious.
How much water?
So once the baby is eating solid foods they should just get some sips from a sippy or regular cup. This is usually around 6 months. The more they eat the more water. They usually don't need more than 2 oz. a day during the first year. Just offer some after they've breastfed and had some solids. They'll let you know how much they need, and so will their poop!