Lessable and disabled

Afraid of water or not such a keen swimmer?
To start, you could sit at the back. Here you are furthest from the action and closest to the guide.Look for a lower grade river - say 2 or 3 to start with. Let your guide know that you have concerns and then they will help you to overcome them and enjoy yourself.

More severely less able
For someone with minimal control beneath the waist or missing a limb, the rafting would need to be stable and secure. North America or the Alps are probably the best places to go as operators offer more options and have a wider range of craft and rivers to choose from. The excellent book The Complete Whitewater Rafter has a section devoted to rafting for the lessable.

The man who led the first run the Grand Canyon rapids had lost an arm in the American Civil War.