You might want to surf to boards.cruisecritic.com It's a large cruise board, which also has sub-boards dedicated to individual ports. All our ports are on there (of course) and you can find a lot of info about different tours, doing-it-yourself-tours etc. etc.
I looked into accessible (read; powerchair) island tours on St. Maarten, but that's like wanting to rob a bank so expensive.

So, grabbing a cab and putting the manual chair in the back will probably work out a lot better. Cheaper and we can decide on the spot where to go when.
Me doing a dive on St. Thomas should work out by the way. Already gotten the most important part of medical approval, so I'm assuming the rest will be a breeze.