Mexico
Best overall months to dive: Jan, Feb, Mar, Apr, May, Nov, Dec
Mexico's diving splits across two coasts and two completely different experiences. In the Pacific, the Revillagigedo Archipelago (Socorro) is liveaboard-only from a 24-30 hour crossing out of Cabo San Lucas, running roughly November to May for giant oceanic mantas, silky and silvertip sharks and schooling hammerheads. On the Caribbean side, the Riviera Maya's cenotes are the opposite: freshwater cavern dives in a flooded limestone aquifer near Tulum and Playa del Carmen, diveable year-round at a steady 24-25C, with light beams, stalactites and haloclines instead of big animals. The two share a country and nothing else — plan them as separate trips.