We have snorkeled in both and BElize hands down is MUCH better. We booked a snorkel trip from RCCL and it was wonderful. The company was fantastic and they took us to a little cruise ship only type island for anout an hour afterwards. We regret we didn't get the time to go into Belize itself but maybe next time. We snorkeled in Nassau and at Coco Cay and while it was interesting it was nothing like Belize. It seemed much more authentic.
We have Costa Maya on the itinerary for a possible Nov cruise and may change the cruise b./c I haven't heard much good about it.But if the rest of your trip is exciting you could go browse and spend the day on the ship.
Carnival Fantasy 6/05
RCCL Sovereign of the Seas 4/07
RCCL Navigator 12/07
RCCL Navigator 11/08
RCCL Liberty 7/4/09