To live onboard 50 weeks a year must be very expensive? Do they stay in same ship?
Comparisons have shown that living on a cruise ship is not as expensive as a senior citizens' assisted living facility. Perhaps for some folks, they can live more cheaply on a ship than paying a mortgage on a house or rent, plus food, entertainment, etc. It won't work if you have school-aged children, but for seniors, it's a choice a few have made.
The two people I've met who live on a ship stay on the same ship, in the same cabin. I'm guessing the cruise line must give them some sort of "volume discount" for being aboard for each sailing.