Cozumel and the Riviera Maya: Reef Season Runs on the Caribbean Calendar
Cozumel's reefs sit inside the Mesoamerican Barrier system, and the water stays swimmable year-round at roughly 26 to 29 degrees Celsius. The practical season is December through April, when nortes ease off, visibility often reaches 30 metres and the drift sites off Palancar and Santa Rosa are at their calmest. August and September are the hottest, cheapest and wettest months, and while trips still run, afternoon storms and occasional port closures on the Cozumel ferry from Playa del Carmen are real risks. May and November are the pleasant shoulder months.
Stay in San Miguel de Cozumel rather than Cancún if the reef is the point: the ferry from Playa del Carmen takes about 45 minutes, and dive operators cluster along Avenida Rafael Melgar within walking distance of most hotels. Prices for simple dive-friendly rooms in San Miguel sit in the mid-range bracket and climb sharply over Christmas and Easter week. If you want reef plus ruins, base in Playa del Carmen and day-trip across. I would book San Miguel between January and March, when the nortes have settled and the crowds have thinned.
Baja California: Sea Lions, Whale Sharks and Cold-Water Windows
The Sea of Cortez is a different animal. La Paz and the Espiritu Santo islands are best from June to November, when the water warms to around 26 degrees and whale sharks gather off La Paz Bay, typically October through February. Cabo Pulmo, an hour and a half north of Los Cabos by road, holds a living coral reef that rewards divers from July to November with calm seas and good visibility; winter brings swell and a wetsuit thick enough to matter. Los Islotes, near Espiritu Santo, is the dependable sea-lion dive and runs most of the year.
Los Cabos itself is convenient but a compromise: dive boats leave from Cabo San Lucas marina, and many sites require a 30 to 60 minute run. I would skip the all-inclusive corridor at Cabo San Lucas for a dive trip and stay in La Paz's centro, where operators launch from the malecón and afternoons are quiet. Getting there means a flight to La Paz via Mexico City or a two-hour drive from Los Cabos airport. Book hotels in either city through us rather than shopping around; the local operators here rarely bundle beds and tanks well.
Cenotes and the Yucatán Interior: Fresh Water, No Waves
Inland Quintana Roo, around Tulum, Playa del Carmen and Valladolid, the cenotes are freshwater sinkholes where the dive season barely matters because there is no swell. Dos Ojos, near Tulum, suits cavern-certified divers year-round; Angelita and The Pit reward deeper certification with haloclines and hydrogen-sulphide clouds. The dry months, November to April, bring clearer light and easier road access, but the caves stay a steady 25 degrees in any month and rain only changes the visibility slightly. Early mornings beat the snorkel crowds at Dos Ojos.
Valladolid is the underrated base: colonial, walkable, and 20 minutes from the Suytun and Samulá cenotes, with Chichén Itzá 45 minutes west. Tulum works if you want beach and cave in one trip, though the hotel strip is pricey and far from town. I would book Valladolid for a cave-focused week and treat Tulum as a two-night add-on. Neither needs a boat, so costs stay lower than Cozumel; expect quiet mid-week stays outside school holidays.
Gulf Coast and Pacific Options Worth Knowing
The Gulf of California coast around Loreto offers the Loreto Bay marine park, calm in summer and best from June to October, with a small-town base that is closer to the airport than La Paz. Puerto Escondido and Huatulco on the Pacific are wet from June to October and clearest from December to April; the diving is less celebrated than the Caribbean but the open-ocean sites are uncrowded. Veracruz and the Islas de Enmedio reefs sit off the port and improve from March to September, though visibility is more variable than Cozumel.
If you only have one trip, choose the Caribbean between December and April, or Baja between July and November, and let the season pick the coast rather than the other way round. Skip the temptation to combine both in a single week; internal flights via Mexico City eat a day each way. Tijuana and Mexico City are the cities most people search for accommodation before flying south, and booking the gateway night early is the cheapest fix for an awkward connection.