What kind of substance aquatic
birds use to coat their feathers?

Marine birds or aquatic birds secrete a greasy oily substance that is mainly composed of  quite unusual lipids. These birds have an organ  called a preen gland also  uropygial  gland located at the back near the base of the tail that is responsible for the secretion of this substance.

The birds apply this oily substance over their feathers by rubbing their bill and head over the uropygial gland then rubbing off the accumulated oil quite thoroughly over the feathers of wings, back and  every accessible part including  the skin of their legs and feet.

This oil  produces a waterproofing effect on the birds' feathers and thus they do not get wet but water simply runs off their surface. 

Due to the amount of pressure