Dry season, December to April, is Cozumel at its most reliable. Days run warm and bright, the sea flattens out, and the diving viz is usually stupid-good. Net it out: it's the postcard, and you'll pay postcard prices. February and March are the busiest, and that's also when cruise days stack three and four ships deep at the pier. Rainy season, roughly June to October, flips it: hotter, stickier, and the afternoons can open up with a fast tropical downpour that's gone in an hour. Hurricane season technically runs June through November, but the part that actually keeps locals watching the radar is September and October, the peak window. Real talk: a direct hit is rare in any given year, but a storm anywhere in the Caribbean can wreck a week of ferry crossings and dive boats. If you book those two months, buy travel insurance and keep your plans loose. One more honest flag: sargassum seaweed can drift onto the windward beaches roughly May through August, and how bad it gets swings wildly by year and even by week, so the southwest beach clubs (the sheltered side) are your safer bet then.