I just got back from Carnival Legend trip, Cozumel, Belize, Roatan, Grand Cayman. We dove in Belize with a local shop, Sea Sports. They were good, but there were 3 on out boat off the same ship that had a 4 PM Ship time last tender. We did have to push the operator to pick up anchor and get headed back to the pier he was in relax mode. Also on our first dive he did have us going to a depth of 130ft, it was a tough call for me, not having AOW, it was go with the group for that depth or turn around on me own with my wife and go back to the boat on our own. I would say go with the ship, although when we were there Carnival cancelled the dive, telling one person it was sold out, and telling another person it was cancelled due to lack of interest.
The 3rd diver that was with us on the Belize dive did go diving in Roatan with the excursion, he said it was it was much nicer than Belize.
Grand Cayman, wife and I did a shore dive on our own through Don foster which was a nice dive. It was the same dive the cruise offered for 69 a person. Me and my wife paid 60 total, that was renting a BCD, REG, and shorty. We did have time to do a second dive for the cost of a tank each, but wife wanted to ship.