I was told no water under 6 months old. It throws off their electrolytes and sodium levels, and can lead to water intoxication. Breast milk has all they need... even on those super hot days. Babies are smart, and know when to drink for thirst or for hunger. The foremilk is largely water, and is really all they need on hot days like that.
L is 7 months, and has never had water yet. We spend a lot of time in the heat, and I just offer him an extra bottle of breast milk.