Cozumel, Mexico (Money Magazine) - Though it's too far to swim to, from shore you can see where the bottom drops out of the sea.

A 3,000-foot channel separates the island of Cozumel from mainland Mexico, and life has blossomed at its edges: coral and sponge, brightly colored fish, even endangered Hawksbill sea turtles.

Below, the blue gradually darkens to black, and with the current pulling you along, all a diver has to do is relax and take it all in as it passes by. On an especially clear day, your view might extend to 100 meters.

If the cobalt-blue angelfish seem bigger here than anywhere else in the Caribbean, it's due to the special character of this reef. Normally reefs are barriers that protect the mainland from ravaging weather, but here Cozumel protects the reef.

Simply put, Cozumel offers some of the world's finest diving at the best price, thanks to this quirk of geography and to the travel promoters who have helped make this small island off the Yucatan peninsula a destination for everyone else.

If you don't dive, you'll still find plenty to do, from the activities you'd expect to find at any beachfront resort to excursions deep into Mexican history. If you do dive, you may be reluctant to surface.

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You can, of course, learn how to dive in Cozumel, but why waste vacation time? Learn at home before you go -- a local dive shop can tell you how.

There are scores of dive outfitters in Cozumel -- most resorts have a shop on the premises -- but while prices are competitive (figure about $50 for a two-tank dive), the operators are not all created equal.

Diving is not without risks, and some shops manage them better than others. Aqua Safari ($55 for a two-tank dive; 011-52-987-872-0101; aquasafari.com) and Scuba Club Cozumel, a dedicated dive resort ($55 for a two-tank dive; 011-52-987-872-0853; scubaclubcozumel.com), get high marks from divers and dive shops in both the U.S. and Cozumel. Both dive shops also have a reputation for treading carefully on the fragile reefs.

Beaches, of course, ring Cozumel, but they're hardly spectacular; many dedicated sun-seekers head to Playa del Carmen on the mainland, an hour away by ferry.

Mexico's most mystical beach may be in Tulum, about 40 miles south of Playa del Carmen, where bathers laze on the white sand beneath a cliffside Mayan city some 1,100 years old.

More high-minded visitors come to Tulum for the ruins themselves, and it's easy in Cozumel to find organized tours to Tulum and other archeological sites in Yucatan, such as the partially excavated structures at Coba.

Trips to the Mayan center Chichen Itza are widely available; it's a daylong excursion by small plane that starts at about $120 per person.

You can find a good room at a good resort from about $100 during winter in Cozumel.

The Fiesta Americana (doubles from $110; 800-343-7821; www.fiestamericana.com) and Hotel Brisas Cozumel (doubles from $117; 877-454-4355; www.brisashotel.com) are popular with divers and tour operators alike.

Hotel Brisas has fewer frills but is within walking distance of restaurants and clubs in town; the Fiesta Americana offers a little more hotel, but is much farther away.

Not quite as far south of town, the Presidente InterContinental is Cozumel's plushest hotel, with prices to match (doubles from $240; 800-327-0200; intercontinental.com).

Despite its being widely packaged, finding the best way to book Cozumel isn't so simple. In September, we checked rates for the first week in February at four hotels from three U.S. gateways, and found that at least a third of the time, it was cheaper to book separately.

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Two exceptions: US Airways Vacations (800-422-3861; usairwaysvacations.com) had good rates for connections through Charlotte, N.C. And tours from operators such as Apple Vacations (applevacations.com) and Funjet (800-558-3050; funjet.com) that use charter flights can sometimes -- though not always -- save you hundreds of dollars.

Apple offered seven nights at Hotel Brisas Cozumel from Denver for about $1,500 (book by Dec. 5), almost $500 less than the next cheapest package and $700 less than booking air and hotel separately.