Swimmers were immense transcendental beings which dwelt in the Void between universes. They were so vast that universes burst against their sides like bubbles against a whale. One and the hermit released the Fendahl Predator to prevent Swimmers from being attracted to the universe. (PROSE: The Taking of Planet 5) They are also known as intercreationals. (PROSE: The Book of the War)
The Eighth Doctor considered Swimmers to be among the predators that could eat him while he was drifting through the Time Vortex unprotected. (PROSE: The Shadows of Avalon)
The loa Baron Samedi, his "wife" Maman Brigitte, and Spinning Jenny were Swimmers. According to Homeworld agent Cobden, Swimmers were merely another form of Leviathan that disturbed the "waters of reality" merely by existing. (PROSE: Spinning Jenny)