They begin at the southeastern tip of the Florida peninsula , about 15 miles south of Miami , and extend in a gentle arc south-southwest and then westward to Key West , the westernmost of the inhabited islands , and on to the uninhabited Dry Tortugas . They begin at the southeastern tip of the Florida peninsula , about 15 m south of Miami , and extend in a gentle arc south-southwest and then westward to Key West , the westernmost of the inhabited islands , and on to the uninhabited Dry Tortugas .