Both comments by HanaS77 and Ms Harrington are certainly valid, but I would like to add a practical point of view, based on our modest cruise experience.
Since 1999, that my wife had lost her leg and recently her vision, we have cruised eight times. The Cruise Lines that I tend to book our cruises, are Princess, Celebrity and RC.
I always book a HP cabin and to date, we did not encounter "Rube Goldberg" bathroom facilities, nor did we ever pay a premium for the HP room. As far as the emergency procedures, we are permitted to use the elevators to our Stations and assemble in a specially assigned area for the wheelchair handicapped.
Again, both my wife and I never have found, as HanaS77 describes - the lack of facilities at open restrooms, restaurants and elevators. More so, we always book a HP cabin with verandah and most of them are right next to the elevators. Oh yes, I also make sure that the bathroom in our cabin has a "drive-in" shower.
I am certainly aware of the limitations that a HP person may encounter on any cruise ship, but with some careful planning (as far as one year ahead of departure,
in our case) it sure has worked out for us, so far.
If I have to include a negative thought, that would be, the emergency evacuation - it does make me a little bit nervous if it were a real evacuation of some 3000 passengers and crew, and secondly , being a retired architect, I know not to hold my breath for any valid ADA improvements for a long time!