On my antenatal course the mw said not to give extra water as over hydrating baby can be harmful, and if you make the bottles correctly they get all the liquid they need. Also, you're filling their tummy with liquid that has no nutritional value which can cause them not to drink as much formula and can cause weight loss. I haven't looked into it, that's just what I've been told. First time mum so I'm just relying on what the mw said.