We just came back from Diving with Blue Magic. We dove several times with a lady that was in her 60's and not capable of managing her equipment easily above water. They were very helpful,many others on the boat will be glad to assist. BM has Sleep and Dive packages staying at Casa Mexicana that were cheaper than anything else we found and included a Great breakfast buffet. The hotel was great and is in the middle of everything. Ask for an upper floor room. I would avoid the all inclusives simply because there is some truly great food in town. The scallops at guidos and the burritos at Burritos Gorditos are must do's. Do Guidos early in the trip...it's pretty romantic and you will probably want to eat there again.
Almost any dive op will work with you to accommodate your needs. Bluextsea is a frequent contributor to this site and deserves a look, Dive With Martin is great, and I have heard good things about Aldora as well.
some of the main differences will be size of boat and where they leave from. DWM is at the international pier and leave about 30 feet in front of their shop. blue Magic leaves from their shop on N 4th and they pay for the cab to Caletta Marina South of town. Not sure about the other two, but those answers are an email away. Some have their own pier,some will pick you up from your hotel, I would avoid cattle boats and stick with the 6 pack boats, especially if you will need some help. Some do the surface interval on the boat, others go to a beach. All of the companies I listed have small fast boats.
Cozumel is very friendly. The trips I've been on people have offered to loan others equipment if needed. I have a very difficult face to fit and another diver from California with a huge melon like mine offered to let me try his mask during the surface interval.
pick three or four dive ops and email them and ask about any special needs you may have. I will bet lunch they will all work with you and the diving will be great.
Congratulations and safe travels,
jay