The ports that dock are wheelchair friendly--I'm not sure after you get into the town, but the port area is. The ports where you tender to a smaller boat (like the Western at Grand Cayman) could be a problem.
That said, there is plenty to do if you choose to stay on the ship.
People I've known have brought their own chairs. THe ship, of course, is accessible.
You could rent one on land in Orlando and bring it to the ship....