My personal opinion... YES, you will be disappointed with snorkeling in Cozumel.. It is IMHO the best scuba diving in the Caribbean, period. BUT snorkeling, nope. The reefs in Cozumel are far to deep to see, and it is all drift diving with current you cannot fight, you have to go with it.
Snorkeling is best done at your own pace, and being able to hang out and watch the fish. Bonaire and Roatan are the best IMHO for that.