For breastfed babies, the World Health Organization (WHO) recommends exclusive breastfeeding for the first 6 months of life, which means that they do not need any additional water during this period. Breast milk contains all the hydration and nutrition a baby needs in these early months, even in hot climates.

Here are a few key points on why water supplementation is not necessary for breastfed babies under 6 months:

1. Adequate Hydration: Breast milk is about 88% water and provides sufficient hydration for your baby, even in hot weather.
2. Risk of Nutritional Imbalance: Giving water to newborns can lead to a condition known as water intoxication, which can dilute the sodium levels in the baby's bloodstream and potentially lead to serious health issues like seizures, coma, and even brain damage.
3. Interference with Breastfeeding: Feeding water to young infants can fill their tiny stomachs and make them less interested in breastfeeding. This can lead to decreased milk intake and potentially affect the baby's weight gain and growth. It can also interfere with the mother's milk production because breastfeeding operates on a supply and demand principle.
4. Risk of Illness: Introducing water to infants under 6 months can also increase the risk of illness from contaminants, especially in areas where water quality might be compromised.

After 6 months, as you start to introduce solid foods, you can also begin to offer small amounts of water to your baby, but breast milk (or formula, if you're formula-feeding) should still be the primary source of nutrition and hydration. Always consult with your pediatrician or a healthcare provider regarding the best practices for feeding your baby, especially if you have concerns about hydration or feeding in general.