Fridges are well stocked with drinks, juices, and snacks. The cabin steward will inventory the fridge everyday and put the charges on your S/S bill. We bought beer in the port and just replaced what we used out of the fridge.
Actually the "fridge" on Spirit Class ships is a mini bar. If you don't plan on purchasing items ask your steward to put the items back in stock and then you can use it for water and soft drinks you bring on board.