Unlike most other cruise lines where food is available 24/7, on MSC it is not. The buffett opens and closes between breakfast/lunch/tea. In the evening it becomes a chargable bistro. Tea and coffee is only freely available during breakfast and lunch, chargable at all other times, as is with water which in the evening they will only sell you bottled water. You CANNOT get a FOC jug of iced water with your meal.
Cabin service available 24/7 if you are really in need of food. I would advise buying one of MSC's drinks packages through your travel agent. Covers water, soft drinks, beer & wine etc.