Personally, we NEVER book excursions through the cruise line...WAY too expensive!!! Cruise Critic is an EXCELLENT place to learn about private company excursions...and also to make GREAT friends prior to cruising, like we are doing here.
We are swimming with the sting rays and doing the Ultimate Dolphin Encounter with Native Way Water Sports in Grand Cayman, a friend of mine owns it.

In Cozumel, we purchased Day Passes to Cozumel Palace...it's an AMAZING resort right on the reef, and we cannot WAIT to go back for the day!
I have been to Maya Chan MANY times...it's AWESOME, however...seeing as how we are only going to be in Costa Maya for less than 5 hours, and it takes 30 minutes each way to get to Maya Chan....we have decided NOT to book this time, as sad as that makes me, because all of the people there are GOOD friends of mine and I would LOVE to see them...just cannot justify the expense for 4 hours MAX....I am VERY disappointed in Disney as we usually have LONGER port times when we cruise on Disney as opposed to Carnival or
RCCL
...this time they are ALL shorter...I don't get it, but oh well...cannot WAIT to sail on the Fantasy!!!