Exclusively breastfed babies do not need water supplements – your body makes the perfect milk for your baby, actually changing the composition as he needs more or less water.
Giving water to newborn babies can contribute to jaundice and slow weight gain: water may fill a baby’s tummy and so reduce the amount of colostrum or breastmilk they get. Water supplements may also stop you establishing your milk supply as your baby will ask to breastfeed less.
Even in very hot weather your baby doesn’t need additional water, as long as you respond to his cues to feed he will get all the fluids he needs in your milk. This article from LLL Canada gives more information about breastfeeding and hot weather.
Our post on starting solids has information about introducing water to your older baby.