I'm not sure if its a bug or a feature... but since the examination quote mentions "deep ocean" here we go:
I'm testing the new shipwrecked beta... at day 17 I just realized that on my starting island suddendly a "Tar Slick" spawned in my jungle biom. Maybe it spawned a few days before, I'm not sure. At the beginning of my game there was definitly no Tar Slick in my starting Jungle.
I have totaly no idea. Maybe there is some coincidence with me spawning the Tar Slick in the ocean near my island in the south by using a Quacken Drill.