We were just on a cruise in May that stopped in Cozumel and used the cruise ship dive op, Sand Dollar. I think they are the only one for all the cruise lines now. So if you have enough people in your group to schedule a whole boat with an independent dive op, you best bet is to go with them.
We were satisfied with them, but they weren't the best we have ever used, and clearly not the worst. They did a good job considering it was raining and current was stronger than normal, vis was lower due to rain and cloudy conditions. I am sure you would get better dives with an independent, but if you can't book a whole boat you are stuck. That was our problem, but turned out better than we expected. We have read such horrible reports on them, but it wasn't bad at all. We were diving all week with the same people (2 ships docking at same ports each day, so some from each ship) and with Cozumel our last day of diving we knew each other and who to watch. We did have one dude with a camera who swam around like a maniac and got low on air both dives. The two DMs went in with our group of 8 divers that day, and one ended up taking him up at the end of each dive while the other stayed with the rest of us. You can get bozos like that on any trip though, not because he was a cruiser. No one in the group was a noob, so we had zero problems diving together... except for the one dude who had to go up early.
Seriously, I would use them again if we decide to dive from a cruise ship in Cozumel again. But I prob won't dive our next cruise there. Cozumel has so many great excursions that I want to do something else next time.
FYI - we just did a week land based dive vacation to Cozumel last month, so we do love divng Cozumel..... its just that we want to do other things there, too.
My dive log:
#1...Santa Rosa reef, 73' max, 32 minutes... shorter dive due to ripping current, unable to enter most of the swimthrus, vis bad.
#2....Yucab reef, 53' max, 46 minutes...excellent dive, 2 nurse sharks, tons of fish, toadfish, southern rays, beautiful color on reef, light current. Vis still low due to weather. Great dive.
robin