If you have a disability and want to go on a cruise, there are cruises available, you just need to do your homework. Here a few things that you will want to look for:
Cabins - are they wheel chair accessable? Or are the doors wide enough to handle wheel chairs?
Itinerary - If you are taking a cruise with many stops, do the tenders accomodate wheelchairs?
Medical - Does the ship you are planning to go on have a doctor? Many do not and only have a nurse on board. Check ahead.
Here is a list of ships that are 'disability friendly':
Princess, Holland America, Cunard, Crystal, Celebrity, Carnival, Costa and Royal Carribean. If you plan ahead, then you will truly have a wonderful holiday.