Divi Tiara on Cayman Brac. We were there in February and there was a disabled diver on the boat with us a few days. Resort staff and dive staff are very helpful. If you need anything special, just talk to Max, the manager. He is super-helpful. I think if you get a first floor room, there are no steps to navigate anywhere on the property. I could be wrong about that, but there sure aren't very many. It's a fun place to stay and dive, the diving is great, and the food is pretty good. The front page of their website currently has a link to a story about disabled divers:
http://www.diviresorts.com/divitiara/resort.asp.