Well, I'm the GF that Flot mentioned. We survived our cruise and had ton's of fun. Even though everything was last minute we got to dive evrywhere. Here's my recomendation:
Cozumel: Aqua Safari was awesome. I booked them online and had no problems. They were very helpful and friendly. I even called them because I was my paranoid self and they laughed at me and told me that everything was fine and not to worry. Just show up at the shop and they even gave me directions on how to get there depending on which dock our cruise ship used. Also, they have several boats and they DM's are experienced. We got to stay down until WE had reached 500PSI. Not the first diver of the group. You simply signal the DM that you are going up and the boat picks you up.
Grand Cayman: Booked through the cruiseship because despite my 50 phone calls to the island, no one could accomodte our ship arrival time. The dive was ok but way to short. The Op was new and I think they were really overcautious. They treated everybody like new divers.
Timetraveler did it right! just go to Eden Roc, grab some tanks, and shore dive from there. It's one of the best on the islands anyway. We dove Eden Roc on a night dive two years ago and loved it!
Costa Maya: Used BOS. Small op. veeeeeery laid back! But good dives & personal service. Turns out we were the only two divers with them that day. So we jumped onto a little panga boat and headed out! I loved it. Besides, so much there is still unexplored that it was cool to do an exploration dive with the DM.
Belize: I also did the cruise ship dive. I agree with Timetraveler that it's the best option when you are tendered so far out. The dive was OK. We used Hugh Parkeys as well. BF didn't enjoy Belize diving very much. But I'm willing to go back and give it another shot. Hugh Parkeys staff was great. We were separated into two groups: experienced and non-experienced. We were very happy about that. Our DM was also very good. You could really sense his enthusiasm and passion for what he did. This means that he enjoyed taking people out and showing them the sights underwater. He really went out of his way to find stuff for us to see. So many times you go out and the DM is just there to dive and not really a "host". But at Hugh Parkeys they truly enjoyed it!
Good luck! I'm jealous. We just got back from Diving PR (la Parguera & Desecheo) two weeks ago & I'm already planning our next dive adventure. CAn't wait... Can't wait....
BF thinks I'm nuts!
but that's oK!!