Thanks to its subtropical positioning on the globe, in the waters of the Caribbean Sea and right next to the waters of the Gulf of Mexico, Cozumel sees ideal weather throughout the year. The average temperature in Cozumel is 80 degrees. And even in the cooler winter months, the temperature rarely falls lower than the 65 degree (Fahrenheit) mark. It is not uncommon for a day in the middle of February to see highs in the upper 70's and sometimes even up to the lower 80's.
Weather conditions for scuba diving are near perfect throughout the year, especially on the leeward (west) coast of Cozumel where the winds are significantly milder than on the other windward side. Fortunately, this side is where the majority of the best reef sites and dive destinations are located. And since Cozumel is home to so many different reef sites, when the weather disallows for diving at one site, there are dozens more to choose from.
The winters in Cozumel are slightly cooler, so make sure to bring a sweater or light jacket. There's a good chance you will not even have to use the sweater, but it's a good idea to bring it along just in case. Likewise, most of the dive shops in Cozumel offer complimentary or rental swim suits for diving when the water has gotten a little cooler than normal.
It is said that the rainy season in Cozumel is from July through September. This is when the majority of rainstorms happen throughout the year, but the thing with rainstorms in Cozumel is that they are very short lived. Most rainstorms last no more than 15 or 20 minutes. Day-long rainstorms are almost completely unheard of in Cozumel. This means that if you wait around for just a little bit, the chances are high that the storm clouds will pass overhead and the weather be back to normal.