San Pedro isn't deliberately wheelchair-friendly, but much of it isn't particularly unfriendly. You need to choose carefully where you stay - my immediate thought is that Sunbreeze would work well for you. The streets are mainly sand, and the beach is also a thoroughfare, so you'd really want a chair with fat soft tyres. But you'll find a lot of willing help for the places where otherwise you'd struggle. I haven't come across any wheelchair-friendly vehicles, though many taxis are minivan style and could work.
If you want to dive and don't have use of your legs, I believe there are one or two local instructors who have relevant training. I do know that there was a wheelchair-bound diver staying at and diving with Ramon's recently.
AmbergrisCaye.com 
