Formula/breastmilk satisfies a baby's hunger AND thirst. Water is not needed and too much can be harmful but a few drops are not going to hurt. Think about it, when you are mixing up a formula bottle, you are never going to consistently put the EXACT same amt of water to the drop. Sometimes, you might put a teaspoon more water and just not realize it, sometimes maybe a teaspoon less.
With all that being said, what reason would you have for giving an infant water? I can see giving a dropper of water to get rid of hiccups or something but other than that, there is no reason to give a baby water.