Dehydrated Ocean
First mentioned in The Colour of Magic, this "anti-ocean" is located in the Great Nef desert. This is an incredibly dry desert, to the point it has negative rainfall. As a consequence of this, the Great Nef is home to the Dehydrated Ocean, which consists of water in an uncommon fourth state (it dehydrates, leaving a silvery mildew-like freeflowing sand), and Light Dams, which dam up and save the Discworld's slow-moving, syrup-like light. It has very strange fish, possibly like those of the Ankh river, another place which has water which is only water because it's less scary than it being something else.