Hi!
I am a full time wheelchair user and just came back from the places you are going.
St. Thomas -- Megan's Bay is easy if you want to do a beach. Just ask your taxi driver to take you all the way in. They can if you are limited in your mobility. There are chairs for rent and a drink waiter on the beach. There are bathrooms but they are up a ramp but have raised toilets and bars. Mt. Top is fun. You can get an accessable tour of the island and Mt. top geared to slow walkers at
www.accessableadventures.com
St. Martin-- we hired a Taxi for the day for $100 Us plus tip. He was great. We went to the butterfly farm which is a bit of a walk from the cab but if you go slow there is a place to sit and rest before you tour the butterflys. Best done in the am.
I highly recommend going to Marigot. The french capital is lovely. Very managable to walk around the flea market have a bit to eat and then tackle more shops if you want. OUr driver took us all around the island and we went to Orient Beach which has a lovely view.
nassau--has been a while since we were their. Hired a cab and went to the beach. You might want to check out Arlantis---a little to crowded for our taste.
Hope this helps. feel free to email me with questions.
Jean