It dependends, if they dock or tender.
We tenderd in Key West but there were many handicapt passengers and the crew really helped them, even with their scooters, to bring them into the tender boat.
In Cozumel we docked far away from the center of Cozumel, so we took a cab. I think, also no problem with a disabled person.
Maybe there are some problems on shore excursions, but there might be someone here on this board who can answer this better than me.