When we went on our cruise, my daughter was almost 5. We didn't do any excursions, though I would have loved to play with the stingrays. We got to see some when we were swimming at 9 mile beach. It was so quiet. Beautiful. There was even a rest room and an ice cream truck with other snack goodies if we needed them. We just had so much fun finding "pirate fingers" (long, skinny coral that we left behind so the ghosts wouldn't haunt us...wink wink) and other treasures.
At Cozumel, we went to San Fransisco beach. It was nice, and the fellows got a treat...topless too! My kids didn't even notice. They were farther away from the main group of people. There is even a pool that you can sit in and drink right at the bar and stare out into the beautiful ocean. There are clean restrooms there too. We got Paris' hair braided (whole head with two different colors of beads) for $25. Her hair is fairly long. Some were asking $50!
Key West, we didn't even bring the kids off the boat. They were tickled knowing we were off the boat and they were "big kids". It worked out great. One of the best hints I was given when we got on the boat was to go straight to the kids pool. You will never find it that empty. The kids had a blast and my son actually learned how to swim under water!
Have a great time!
Kitty