In January, the water temperature in the Florida Keys typically ranges from 70°F to 74°F (21°C to 23°C). For many people, this is a bit too "bracing" for a long, leisurely swim, though it is still much warmer than the ocean in the northern U.S. during the same time. The air temperature usually hovers in the low 70s as well. While "Polar Plunge" enthusiasts will find it refreshing, those used to the bath-water temperatures of August (which can hit 90°F) might find the January water startling. If you plan on snorkeling or diving at the reef, a 3mm wetsuit or a "shorty" is highly recommended to prevent your core temperature from dropping during extended periods in the water. It is also worth noting that "cold fronts" can occasionally push these temperatures down by a few more degrees for a day or two. If you are staying at a resort, you'll likely find that most pools are heated to a much more comfortable 82°F during the winter months to compensate for the cooler ocean.