Большой китайско-русский словарь
yú shuǐ zhī huān
(ee shway jhh hwun-zhī is tricky: say “shh” while your mouth is in the position for making a j sound) A euphemism for sex in use since ancient times. Literally, “The fish and the water are happy together,” from the play Romance of the West Chamber by the Yuan dynasty play-wright Wang Shifu (1260-1336).