Do not waste your money on bottled water while in Rome
Tap water is drinkable everywhere in Rome, and it comes from a fresh water system (actually inherited from the Ancient Rome Aqueduct System) . Rome is plenty of small running water small fountain (the so called “Nasoni”) . So do not waste your money buying bottled water, it is strictly not necessary.
Another consideration about water has to be done when you are sitting at the restaurants. There the situation is different. Even if the restaurants would have the same tap water you drink in the city, they consider “rude” (do not ask us why, but it is the customs) if a client ask them for tap water. So be ready to pay for the bottled water when you eat at a restaurant.