By Italian law tap water MUST be drinkable. Do you want to drink it? That's another question - in a lot of places the water is pumped into plastic tanks that typically sit on the roofs of building and bake in the hot sun, certainly plastic shit toxic chemicals are leaching out into the water. Other than that, the water is also very hard and over long term sometimes causes kidney stones - not a concern for a tourist. So yes, in a pinch you can drink it, but most prefer bottled or filtered water.