It it safe to drink the tap water in the Maremma (Manciano) Italy or should we stick to aqua mineral no gas for drinking?
Why shouldn't it be safe? It may taste differently than what you are used to, but all EU countries must comply with the same rules.
Ask your host. Tap water in towns has to be drinkable or they put notices up. If you're staying somewhere in the countryside it might be off grid with a well or something. Restaurants will expect to bring you mineral water and will think it odd if you insist on tap water. Some now do filtered tap water (so you still get to pay for it!)