In our experience, (my wife is an amputee), there are Lines and newer vessels that have
better accessibility in most areas. I am saing "most" because to my knowledge, there are
problems with the deck with swimming pools. There are railings to hold on to, but there are either steps down to the pool area, or more steps up- to the hot tubs. At times it is impossible even to navigate the WC, because many move their lounge chairs and block the isles.
As for your comment, RE scooter, we have given up on my wife's scooter and only use a foldable WC. I plan our cruises, obviously by destination, first, BUT then I search through
ships' deck plan, where is the best location for the HP room and call the Line if it is available. Only then will I book the cruise. There are a few "sister" ships, example our
previous cruise on Celebrity Summit - I've booked the same ## cabin on Celebrity Millenium for our next cruise, March 12, next year. So, I know what is in store for us ahead of the cruise.. Scooters, I've found are OK at home and for the shopping, but Airlines are not "scooterfriendly", and once you are on the ship, they will not let you on shore with your scooter, not to mantion, where to store it in your cabin.