The Rhoynish water-wizards were powerful worshippers of Mother Rhoyne.
During the Rhoynish Wars, Mother Rhoyne was said to whisper the secrets of the Rhoynar's enemies to them, allowing the water-wizards to raise walls of water with which to drown their foes. The water-wizards used the same magic to raise enormous water spouts to defeat the dragons of Valyria.
In the books
Despite not being a witch, Queen Nymeria was called a "witch queen", as it is said that when she brought her people to Dorne she had Rhoynish water-witches whose secret spells made dry streams flow again and deserts bloom.