I have not given water to my baby because he's still in my tummy, but as a member of the Breastfeeding Pinays group, I've read that too much or excessive water may cause water intoxication among babies. Some scholars say that even a few mL of water may affect blood sodium in a baby's body. This isn't good because it leads to electrolyte imbalance. Even a slight change in a baby's electrolytes is detrimental for his/her health as it may cause tissue swelling.
I've read a lot of Facebook posts about this and I personally haven't tried it. I asked our pedia and he said that babies still need water, on top of formula/breastmilk. I just follow his instructions and my kids turn out to be fine.
I didn't give my daughter water until she started eating solids. No vitamins, no nothing aside from breastmilk. She's 2 years old now and very healthy.