Welcome aboard, Tammy! I'm thrilled that you're asking this question, because I just disembarked the Magic on Friday, from our 6-night Western Caribbean sailing, and had the opportunity to try a couple of great Port Adventures in Grand Cayman and Cozumel.
I highly recommend that you do the
Boatswain Adventure in Grand Cayman. I was there last Tuesday and the Turtle Farm and dolphin area there are absolutely amazing. My favorite part was getting in the water with the sea turtles, getting to hold them and exploring the entire area. I can't recommend it enough. Most Port Adventures for me are a "do it one time" thing. But, I will go back to the Turtle Farm every time I visit Grand Cayman from now on, it was that wonderful! We had lunch at Schooner's Bar and Grille, and if you do this, I highly recommend the Conch Burger. Delicious!
As for Cozumel, I haven't done either of those three adventures you mentioned. However, I can highly recommend the
Cozumel Ruins and Beach Tour. This was a great blend of beach fun time and learning some history of the area. If you are set on snorkeling though, just go with whichever tour includes more of what you're looking for. If you go with one of Disney's sponsored Port Adventures, you really can't go wrong. Disney takes care of all the details and ensures that even if an adventure runs late, you will not miss the ship.
Have fun planning the rest of your cruise, Tammy!