I think that's less of a concern than the constant medical care usually required of an elderly person. On top of the cost of a cruise are you going to pay to have a nurse on board with them at all times? How about if they need regular medical supplies. How would someone get say, insulin, delivered? What if they need dialysis? If there is senility involved, a boat is a bad place to be.

A senior who can get by living on a cruise ship is probably also healthy enough to get by living on their own. I think it really only works in cases where the person is healthy and mentally all there, but needs someone to cook, do laundry for them, etc.