The Tar Slick is a naturally-spawning object exclusive to the Shipwrecked DLC. It is a source of Tar, which must be extracted by building a Tar Extractor. A Tar Slick doesn't run out, and can't be destroyed, making Tar a renewable resource. A Tar Slick can also be created at the player's will using a Quacken Drill.
The Tar Slick was added to the Shipwrecked DLC more than a year after its official completion in an unnamed beta update.