The Ocean of Ooze was a body of liquid on Skaro that separated the continents of Davius, the original home of the Thals, and Dalazar, the original home of the humanoid Daleks. (COMIC: Genesis of Evil) The Ocean also contained the mist-shrouded, unexplored Forbidden Islands and the Island of Moving Mountains, which constantly changed shape. (DAN: The Dalek Book)
Behind the scenes
The existence of Davius, Dalazar and the Ocean of Ooze seem to be contradicted by The Daleks and Genesis of the Daleks, in which Thals, Kaleds and Daleks inhabited the same continent.