I was told by the midwives here in Australia (heat waves hit the forties) that as long as mum is hydrated (drinking 2+litres of water a day - a big glass of water every time baby feeds and at your mealtimes), then breastfed babies fed on demand under 6 months don't need extra water, they'll get enough hydration from the milk - don't be surprised if they do more frequent shorter feeds as the first milk is the thinnest, hydrating stuff.
Bottle fed babies under 6 months should have bottles slightly more diluted than usual, or extra water.
over 6 months you should be offering extra water at the same time as you offer solids.