Seriously, you could do a lot better if you make your own arrangements.
diveborg is right. Check out on your itinerary when you arrive in Cozumel. I did the RC cruise that goes to Coz, Cayman, Costa Maya, and Belize last spring. We used the cruiseship dive excursions for Cayman & Belize because the cruise ship is tendered there and the arrival times were too close to the local dive boat times. (I researched them all online and called many of them). BUT in Cozumel and Costa Maya we docked at the ports and arrived at 7am. PLENTY of time to get to any dive boat. So we booked directly with the local dive ops after I did my research. For what it's worth, we used AquaSafari in Coz. They have a fast boat and a slower bigger boat. You want the fast boat. But it does sell out so book as soon as/if you decide. They have a real phone number that they answer and can book you. We had two DM's in the water. It's definiely drift diving so, yes, you "follow the leader". But what I really enjoyed was that you follow the leader until you you are at 500psi, signal the DM, pop up and there is the boat. It was well organized but not over controlled where you have to come up even though you may still have half a tank of air! Also, one of their shops is located right outside punta langosta (one of the piers where cruiseships also dock) Our ship docked at the international pier but its a $4.00 5min cab ride to and from punta langosta...