Gill

Gills were a physiological feature which allowed a creature to breathe underwater.

Careshi developed gills at a certain age, allowing them to live in the oceans of their planet instead of on the land. (PROSE: The Suns of Caresh) Marshmen had gills through which they breathed air. (TV: Full Circle) Silmarillions also had gills, and the Eleventh Doctor, when drowning, hoped that he wouldn't regenerate into 'something with gills', like them. (PROSE: Dark Horizons)