Cozumel with a Large Group?

[AlaskaGirl6]

We're going to be traveling on Norwegian with a large group of 13 people. Ages 1 - 65. 

This will be everyone's first time visiting Cozumel. We aren't loving the excursions available from the cruise line at this location. 

We're tentatively thinking that Harvest Caye looks like a good opportunity for a beach day, so we don't really want to do *another* beach day (Playa Mia etc..) but we would maybe like to do snorkeling and/or a glass bottom boat (if we could combine both, that would be ideal since we have some who can't/won't snorkel, but many who want to give it a shot). We've also discussed possibly checking out the museum, which looks like it's within walking distance from the port?  

What should we do in Cozumel?

 

(Also ftr, we're planning Gumbalimba in Roatan, and Mayan ruins in Costa Maya.)