My mother wants to give our 3 month old a little water in this heat

Pune has been very hot this week and amma, who is staying with us for a while, is sure the baby is thirsty. She keeps asking to give him a few spoons of boiled water. Our paediatrician told us milk only till six months and I go with the doctor, but saying no to my mother ten times a day is wearing me down. How did you handle this without it becoming a fight every afternoon?