Because we're so far south in the Keys, many people can't understand why our tempratures aren't significanly warmer than they are. The info below from Fla-Keys.com explains why.
The Florida Keys' unique geographic location has imparted the region with moderate temperatures year-round, providing the perfect climate for favorite visitor activities - outdoor recreation and late-night partying.
Common sense would dictate that daytime temperatures in the Florida Keys, the Southernmost region in the continental United States, would average significantly higher than in the nation's northern climes. Surprisingly, this simple geographic rule does not necessarily apply.
The island chain that makes up the Florida Keys is isolated between the waters of the Atlantic Ocean and the Gulf of Mexico. Here, nature's own air conditioning - cooling breezes sweeping off the ocean waves - keep the Key's climate surprisingly temperate.
Source: http://www.fla-keys.com/breezy.cfm