I shudder at the thought of a cruise - it seems too much like a floating prison with a bunch of strangers and no freedom to set your schedule in port. Boat leaves at XX time and you are on it or not.
However, the one area I think cruises really work well is for people with disabilities. My parents take a lot of cruises for this reason - 1 parent is disabled, 1 is not. The ships are accessible, no narrow walkways, inaccessible museums, lots of walking, cobblestone streets, inaccessible public transit, hotels with no elevators, etc, etc. There is always stuff to do on the ship. My able-bodied parent goes on the excursions to do stuff like horseback riding and kayaking the other one can't confident that the other parent is being left behind somewhere there are things to do and people to hang out with. It seems like the excursion trips available are very descriptive about the ability level required to do the excurion. No showing up at a sight only to find out there is 100 steps involved, etc.
My one exception to the no-cruise rule for myself is that I would love to go on a cruise where I can look out and watch glaciers calving. That can only be done on a boat.