If you're in Italy, don't worry about paying ridiculously high prices for bottled water. Instead, bring your own bottle and fill up at the one of many ANY fountain in Italy. Because yes, they are all (mostly...) safe drinking water.
If it's in the Vatican, does that automatically make it holy water?
Please be prepared for bottle leakage. I suggest having bandaid's on hand. Not terribly effective, but you can still pretend.
Of course, not ALL fountains in Italy are drinking water....