Just off the coast of the Mexican mainland, the porous mass of limestone that arches out of the water has over time become one of the world’s great ocean playgrounds. But Cozumel is about more than ethereal beaches: the island is ranked among the world’s premier dive and fishing spots. And a mere ferry ride away you can explore centuries of human splendor in the Mayan ruins at Tulum, spectacularly sited on the sea cliffs.