Why Does Oil Not Mix With Seawater?
Oils and fats do not have any polar part. This being said, in order for them to dissolve in water, they would have to break some of the water's hydrogen bonds. Water (H2O) most often doesn't allow this to happen, and hence the oil is forced to remain separate from the water.
CLIFF NOTE: An interesting part about oils and water mixing together is most often the oil forms layers in the water based on the oil's density, and when undisturbed, it often stays in place. Pretty cool!