This is all extremely interesting re MDR water and bottled water.
I'm going on the Euribia in just over 2 weeks from Southampton (first MSC cruise) and I'm booked through MSC USA. I had noticed that on my cruise ticket right at the top under "SERVICES ADDED TO YOUR CRUISE" is "MINERAL WATER AND COFFEE IN DINING ROOM".
I found that very strange as we always got coffee and water (not necessarily mineral water) in the MDR on my other cruises. I now understand why.
As it happens, we also booked the drinks pkg + wifi promotion so I presume we can get whatever water/coffee/drinks we like anyway.
Maybe some of the people the previous posters saw with bottled water had drinks packages, rather than an extra amenity?
In any case, we are cruising with family in another cabin and in order to be able to sit at the same table we all have to have the same drinks package.
Maybe when matching 6 tops and 8 tops they only match cabins with the same drinks packages and/or amenities, so that's how the waiters know ?