Scuba Diving and Snorkeling
The rainy season off the Yucatan is from September to November, so expect overcast skies and at least a little rain just about every day. This is followed in December by the arrival of the Nortes, or north winds. These make the sea rough and brings chilly air from the sea over the island. The result is that September through December are not ideal months for snorkeling or scuba diving, as weather conditions will more likely than not make the seas choppy and visibility poor.
Fishing Tips
The conditions for fishing in Cozumel depend on what you came to fish, as there is no time of year when all of Cozumel's sport fish species are present in the local waters at the same time. Barracuda, bonito, mackerel, sharks, tuna and wahoo swim in the local waters all year around, so on a certain level it is always a good time to fish in Cozumel. However, amberjack are absent from April to August, and grouper from April to July. Snapper disappear in May and June. On the other hand, mahi mahi, sailfish and marlin are away from September to February.
Hurricanes
Autumn is the hurricane season for the Western Caribbean, so in addition to the generally poor conditions due to the rainy season, it is possible that a major storm will blow through the area. There's no way to predict the coming of a hurricane, of course, but it is something to keep in mind when choosing the timing of your trip.
General Climate
The climate of Cozumel is thoroughly tropical. Expect the sun to be strong and take the proper precautions. Use a sunblock of at least SPF 20 when you are out enjoying water sports or time on the beach, and wear light clothing that covers a lot of skin to avoid unnecessary exposure. Divers and fishermen in particular should bring a good sunhat to protect their faces during the time they spend outdoors and on boats.
Drinking Water
View the water in Mexico as being semi-treated at best. Unless you are staying at a 4- or 5-star resort that advertises purified tap water, either stick to bottled water or plan on having to chemically disinfect tap water yourself. Many Mexican families use household bleach to kill microorganisms in their tap water.
Eight drops from an eyedropper will thoroughly disinfect a gallon of water after 45 minutes. You may wish to use iodine or chlorine tablets from a camping goods store. In that case, simply follow the manufacturer's directions.
