oil slick 
noun
A film or layer of oil floating on an expanse of water.Four miles from Kilkeel off the coast of County Down in Northern Ireland, there is a fresh oil slick on the water....
- Just when it looked like it might be safe to go back in the water, an oil slick washed up on the beach.
- At approximately two o'clock in the afternoon the PSC fishermen and their boat captain noticed a large amount of debris and an oil slick in the waters where they were fishing.