You can start introducing your baby to water at around six months, when they start to eat solid food. Before this point, your baby will get all the hydration they need from their milk.
Weaning is a gradual process and adding water should be too. Your baby can start with just a sip or two from a free-flow cup (without a valve). This will encourage your baby to learn how to sip and is better for their teeth. Keep up the same amount of milk as you were before.
In hot weather, you may be worried about if your baby staying hydrated enough. If your baby is below six months and is exclusively breastfed, then just offer more feeds than you usually would.
If your baby drinks formula, you can offer them a few sips of water in hot weather. This shouldn’t be straight from the tap as it’s not sterile - instead, boil the water first and then let it cool down. Once your baby is six months old, you don’t need to boil the water anymore.