Oil water separation is actually really hard, it's been a huge problem for over a century. There's a lot of research being done on the topic, dozens of teams from top universities around the world working on it; just look through recent issues of Energy & Fuels, for instance.

Luckily for the environment, it is one of those cases where "more oil emitted into waterways = less money earned", through less amount of product, or less valuable product - cleaning up the oil and cleaning up the water often go hand in hand.