Okay, corny as it sounds, Grand Caymans stingray city is amazing. An hour on the bottom interacting with and feeding stingrays? I’ve been to quite a few places but having a huge stingray put his "nose" against mine and look me in the eyes is pretty thrilling and is right near the top.
You can’t go wrong with Cozumel, but I would not do Belize because if the cruise ship docks in Belize City, you will not have access to the best diving. I love diving in Belize, but don’t think you have the time on the cruise’s shore excursion to reach the better areas. If the ship docks toward the south - Hopkins, Dangriga or Placentia - go for it! But I’ve dove out of Belize City and it’s a long, rough ride to mediocre diving.