Fortunately, the dress code here in the Keys is casual, major casual:) Cotton Tees, sundress, shorts, sandals- you are good to go. For evening, you may want a long sleeve Tee, shawl or light weight jacket. In the evening, our temperatures drop and the breezes kick up starting at about 4pm.
For the months of January, February, March, the temperature have been known to go down to the 50's at night. So long pants and jacket or light weight sweater can be nice. However, the Keys have a mild tropical-maritime climate where the average summer and winter temperatures rarely differ more than 10 degrees.
Even though our Island is the largest of the Florida Keys, it is still pretty small and at some points narrow. With this in mind, it never ceases to amaze me how drastic the weather can be from ocean side to bay side: the change in the amount of wind or rain. It can change within minutes as well.
Yes, there is a hurricane season. Fortunately, the chance of your vacation being disrupted is very low.
If you ask a true Floridian, when Winter hits in January, they pull out parkas, winter hats, scarves, and....oh my...yes.....socks too!