I've gone to Cozumel the last two years during November and have really loved the weather. Two years ago, we went at the end of November into early December. The weather was ideal with daytime highs in the low 80s and cooling off in the nicely in the evenings. The water was between 79 and 80 degrees and vis was outstanding as usual. Last year, we went in the early to middle part of November and the weather was just starting to cool down a little with the day times in the low to mid 80s and nights in the low 70s. The water was still very warm at a toasty 84 degrees, which I understand is a couple of degrees warmer than usual for that time of the year.
Both time the crowds in town were smaller than I have seen during peak seasons and the prices were outstanding on flights and hotels. Last November, Funjet was booking the Hotel Meson at $9 per night including tax. Imagine staying the whole month for under $300.