Cozumel has three serious shore-snorkel venues on the west coast and they are genuinely different.
The Money Bar, kilometre 6.5. Closest to the piers, the best restaurant of the three, the largest operation, and the best set up for people who need help — the facility is unusually level and staff are used to assisting at the entry. Busiest when excursions land. The Dzul-Ha reef under it is the deepest and most varied of the three.
Sky Reef, kilometre 10. A little further out, a little quieter, reef a few metres off the shoreline, and an all-inclusive pass available if you want one. The middle option in every sense.
Playa Corona, around kilometre 9 to 10. The smallest and simplest — a family-run palapa restaurant, a ladder, a reef and very little else. Cheapest, least polished, and the best of the three if what you want is to be left alone.
None of the three is a substitute for a boat trip to Palancar or El Cielo. Shore reef is shallower, smaller and more worn than the offshore sites, because thousands of people a week swim over it. What it gives you is access with no boat, no schedule and no marine park wristband — and on a tight cruise day that is worth more than the better reef you did not have time to reach.