QM2 uses treated sea water for potable water, which I'd heard has higher salt content than tap water on land. At the time, I had hypertension and was prone to feet/ankle swelling when I'd consumed a lot of sodium, plus I went to the fitness center every other day and there was really no way to fill a water bottle other than to hold it under the tap in the bathroom. So I brought nine liters of bottled water on board with me for a 7-night crossing.

Not sure if that would have qualified as "medically necessary" in the eyes of NCL, but I'll just avoid the issue altogether by not sailing with them.