i googled it because my bfs family said the same thing but it turns out you can mess with your babies electrolyte count something like that which isn't good at all , among other things u can Google it and I chose not to listen to them aha
my mom told me to do the same, & I would give my sons some water in between feedings.. its fine i believe, as long as it doesn't substitute for feedings.. it didn't do my kids harm, but every one has different opinions on it.