The Yem-Rúkha were a people from Haradrim Legend.The Haradrim believed them to be a race of Underwater-Humanoids with magical powers.In truth the Yem-Rúkha were a very secretive and secluded branch of the Drúedain or a tribe closely related to them, which had split off from their kinsmen in the first Age and turned south.They did not really live under water nor were they able to breath water, but they did indeed live near the great Sea, on islands and along rivers, in caves often hidden by waters - and they were exceptionally good swimmers and divers and their tribal magicians indeed knew spells and ways to make artifacts which enabled to stay under water for exceptionally long time and other kinds of water-related nature-magic.
The Desert Rukhim were their closest relatives.