Hi Boneman. Welcome.
I can only speak for Cozumel and Belize.
The snorkeling in Belize is wonderful. I've snorkeled at Goff's Caye. Beautiful reefs, tropical fish, crystal clear waters, all set around a tiny island.
In Cozumel, I have snorkeled at Chankanaab Marine Park several times. I haven't been back since Wilma came through. Many on this site have stated that it hasn't been the same since. The snorkeling, as I remember it, was excellent.
I am sure you will get many more responses offering advice.
Have a great cruise.