Mystery slick off Puerto Rico, 6 miles off SW corner of Puerto Rico

At about 1600 ADT on April 25th, an overflight reported a 12 mile slick oriented N to S and about 5-6 miles off the SW tip of Puerto Rico. Reported coordinates are 17d 54.7'N by 67d 18.2'W for the northern tip, and 17d 43.6'N by 67d 18.2' W.Photos show a brown substance in the water with no sheen. Winds were 5 - knots.