1) In Belize we were going to do the same excursion you are doing but hubby really wanted to see the ruins so since he let me choose everything else, I'll give him this one!
2) In Costa Maya, we're just doing the beach thing, hanging out. Probably doing the open bar thing - have to celebrate/drown-my-sorrows for my birthday depending on the mood!
3) In Grand Caymen, we were going to do Captain Marvin's snorkelling tour but it's $34 for 2 1/2 hours so I've looked into the one you mention and have requested more info from them.
4) In Cozumel, we're going bar hopping and then shopping. I figure we'll hit a few good bars and when he's good and "relaxed", I'll get him to escort me to a few jewelry shops! Hee hee! I'm trying to convince him that I should get something shiny! (grin)
Should be a fun-filled trip!