Part of the problem, I think, is that people on the cruise lines who deem a trip "accessible" don't always understand that a 4-inch step can make or break an excursion!
My husband, who uses a power chair, could not go over a 4-inch step ...
He has learned that it is better for him to stay onboard and enjoy the ship while it's in port. But he has visited most of the ports in the past, when he was able to walk, so that's why he's content to stay onboard!
As for Key West, the Conch Train people had us ride the Trolley they operate. I believe that is accessible, while the train is not.
If you visit their Web site [don't have it handy, but you can do a search] you can find more info on accessible rides, etc.
You can also order your tickets in advance, and get a $2 savings [at least that was true in Feb., when we went]
When you get to KW, you have to go to the Conch Train/Trolley booth and explain that you need an accessible trolley, and they will order one to be brought up.