There are two ways to snorkel Cozumel, and they are not the same trip. From shore, beach clubs along the southwest coast like Money Bar and Dzul-Ha have reef close enough to swim to from the sand, which is perfect for a cheap, do-it-on-your-own afternoon. The reality check: entry can be rocky, the current picks up some days, and there's a boat channel you need to stay clear of, so go on a calm day and keep your head up. By boat, you reach the reefs you simply cannot swim to, including Palancar on the south end, the classic Cozumel reef with the swim-throughs and the healthy coral everyone photographs. El Cielo isn't a reef at all, it's a shallow starfish sandbar, and it's boat-only too. Translation: shore snorkeling is cheap and flexible but you're limited to whatever reef is within kicking distance, while a boat trip costs more and runs on a clock but takes you to the spots Cozumel is actually famous for. One more thing the cheap listings skip: the south end sits inside a national marine park, so a separate park fee can land on top of your trip, and they usually want it in cash.