Hi,
The Xel-Ha excursion sounds really fun. My only hesitation would be that you have to take the ferry across to the mainland, then a bus to Xel-ha, then back again. That would take a long time. I think all of the Beach Clubs on Cozumel are a wonderful way to spend the day. Or rent a Jeep and drive around the island, stopping wherever you'd like along the way. Enjoy and have fun!