Water-breather
Water-breather was a term used by Leonard McCoy referring to anyone who used gills to breathe underwater, such as James T. Kirk and Spock after surgo-op, or the Aquans who performed said feat. 

McCoy stated that this mutation was caused by an unidentified substance or hormone found in the bloodstream that affected their entire metabolism. ()

See also 
 Air-breather
 Mermaid
 Xindi-Aquatics

Slang
Lifeform classifications