I did the Sea Trek helmet dive in place of snorkling. It was not bad. Once at the platform we were offered a chance to get our money back as underwater visibility was reduced. One couple took up the offer and returned to the ship.
The head man on the platform said that they have had disabled people and have successfully gotten them to do the dive. In my case with my daughter who would love the experience I do not know for certain that it would work since there is a ladder involved. I honestly do not know on this activity or others for that matter that you can really evaluate them before hand. I have called the cr5uise tour desks in advance but they really won't promise results which is frustrating to be sure. The biggest factor overlying any activity with a physically challenged individual is that virtually noone today will help due to the liability if either the guest or employee gets hurt.