I am just back off the Carnival Freedom.... here is the issue I see.
1. It all depends on your port time in Cozumel. Our ship was not scheduled to dock until 10am, which was 11am in Cozumel. None of the dive ops I contacted were going to have a boat going out until afternoon, and they all needed 4 or more divers to confirm us. This was a real issue for us. Also, we were told the ship would dock at Puerta Maya (one of the 3 docks in Cozumel) but we found out the night before we arrived that the ship would dock downtown at Puerta Langosta. We had already decided not to dive, but if we had booked this would have been a total mess for us.
SO.... what I am telling you is ---- check your port arrival time and dock. Make sure before you book in Cozumel that you know both and that THEY know both.
There are dozens of dive ops that can take out divers from a cruise ship. You are best served to find at least a group of 4 before you try to book. That way you can tell them and what time YOU want to be picked up and go out. This is really important.
2. All diving in Cozumel is great. Period. You do NOT need to go out to the more advanced sites, like the south tip of the island. We dived this past week in Grand Cayman and were really disappointed in it. It did not compare to the incredible reefs or fishlife of Cozumel. NOT EVEN CLOSE. I highly recommend for your first dive to request any section of Palancar Reef, my favorite is Palancar Bricks. Incredible reef, just breathtaking. After that, they will move up north and do one of the shallower reefs, any will be fishy and worth it. I really miss all those gorgeous sponges and color on the reefs. Like I said, Grand Cayman was just okay diving, not as pretty or fishy as ANY dive in Cozumel.
As far as diving in Roatan.... the cruise ships all use Anthony's Key Resort. From what I have been told, they do a great job and all the diving in Roatan is pretty easy. We have been there twice on land-based trips (CCV resort on southside) and love it.
Belize.... I have heard so many negatives about diving there from a cruise ship. The ships dock so far from any good reef that you spend a great deal of time traveling to and from the dive sites. Also, the dives they take the cruisers to are not the best dive sites. Since Belize is a tender port, I think it is better to use the cruise excursion to ensure you get back on time. Just my opinion.
robin