OK, I can answer my own question now. Got back from the cruise yesterday, had an absolute blast. We decided on Cozumel and Belize for the dive trips mainly because we wanted to check them both out for a potential vacation next year.
I wasn't that impressed with the diving in Cozumel, not much better than Key Largo in our opinion. Belize was very good, and I would definitely like to dive there again in the future.
The Costa Maya stop was replaced by Roatan because of hurricane damage but we had already booked our dives when the change was made. We did go snorkeling in Roatan, and the coral and sea life was far superior to both our dives in Cozumel and belize.
After taking to the boat skipper in Roatan we have decided that Bonaire will be our dive holiday for 2008, plus a drive down to Key Largo.
BTW - the Belize dive was great. The dive boat picked us up at the ship. We had a 20 minute ride to their Island/resort to pick up gear, and a 20-30 minute ride to the dive site. The dive boat was excellant, very professional and we had a 44 diver boat for seven of us. They dropped us back at the ship and we were off the ship for a shorter time than in Cozumel.