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)
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.