Views on Plain Water for LO

Hi parents! what are your personal views on giving your LO <6months? ive read it that it is dangerous and i caught my MIL giving water to my 1 month baby 🤯🤯 we keep emphasize not to give but she is those old school type said that we shouldnt believe dr and nothing had happen so far (she claimed all the kids she took care before were well) my concern is.. once i start working, my LO will be under her care. im in dilemma rn..i understand we too drink plain water back then in the olden days..but after doing some reading, i just dont want to take the risk. afterall its my child not hers. jialat la like this.