Does your baby need water?

My son is 5mths old and i have never given him water. the early child hood nurse says it is in the milk so they dont need it yet my grandma and my stepmother gives him water when they babysit him..am i doing something wrong? I have tried to give water to him once or twice but he seems to choke on it every time when its fromt he bottle and then he'll cry hysterically.. even if its after a feed and he is full...