The lost home world of Tethys the sea-troll, who lives on The Circumfence, and every night contemplates jumping off to see if he can find Home again - but he's never found the courage to take that last step and leave the Disc, where at least he has a congenial job...
On Bathys, Tethys worked as the opposite of a fisherman. He was one of the hardy souls who braved the dangers of the uncharted Inland, taking his land-ship and crew out in search of the great shoals of buffalo that roam the land, and which could be caught and brought back to Seas again, as food for hungry sea-trolls... one day, his ship foundered, went out of control, and precipitated over the Edge of his homeworld, whereupon the water-based lifeforms froze solid and drifted in deep space for untold thousands of years. Until one day, the lump of ice which was Tethys was drawn into the gravitational field of a new Discworld, and drawn down into the great ocean there, where he awoke, near to an island called Krull.
The dilemma of Tethys is explored in The Colour of Magic.
The prefix "Bathy-" means either "Deep; depth, as in batholith"; or "Deep-sea, as in bathysphere". So it's quite an appropriate name for a water-based planet.