January historically has the coldest average water temps for the state of Florida. If you are dead-set on snorkeling (or other water activites) you should come prepared for cooler temps.
According to NOAA: Current water temp in Key West 69.8 F
Here is the site:
http://www.nodc.noaa.gov/dsdt/cwtg/egof.html
Have a great trip, and come back to post about your experiences.