At the northeastern corner of the Yucatan, Isla Holbox attracts tourists from late May to October, when scores of whale sharks feed and mate in the waters. These (totally harmless) behemoths are among the largest fish on earth at 15 to 50 feet long, and via this boat tour, you’ll be able to view these gentle giants and even have the once-in-a-lifetime experience of swimming with them. Make reservations ahead of time.