Welcome aboard, Denise! While I'm not a scuba diver, I was just on the Disney Magic for a 6-night Western Caribbean cruise last week and met some friends who took the
Certified Scuba Tour in Cozumel.
They loved it, and I hear that the tour takes you to the second largest reef system in the world, so that sure sounds like a great place to dive.
I've also heard great things about
Costa Maya's Scuba Dive and
Grand Cayman's Shipwreck Snuba tour.
Either way, if you choose to go with a Disney Cruise Line sponsored Port Adventure, you can rest assured that you will have a great adventure and know that you don't need to worry about anything, as Disney takes care of all the details from transportation, adventure, to ensuring the ship doesn't leave without you, despite any possible delays in the tour.
Have fun planning, Denise! Let me know if you have any other questions before you set sail!