It really depends on what you are planning to do on shore. We were on the Western in October and DEFINITELY used repellent when we went to some Mayan ruins (Due to hurricane Ivan, we were at Costa Maya). If you plan to stay right in town at Key West and Cozumel, you probably don't need any. In tropical areas, if you are going into a more "natural" area, it is probably a good idea.
pdarrah