(idiomatic) "a fish who's found water" → In one's element, a duck to water.

Alternative spelling of 鱛

a Japanese giant salamander

“a hundred days of sermonizing, one fart” → a metaphor for how something that one has put great effort into can come to nothing after even a slight mistake, much as a hundred days of serious spiritual study can be lost to the winds

(mahjong) winning off the final discard before reaching the dead wall, worth 1 翻 (han, “double”)

fishfinder

(figuratively) being carefree and unaware of an imminent danger

(figuratively) larger-than-life characters do not associate with unnoteworthy characters, nor do they concern themselves with trivial matters / (literally) boat-swallowing fish do not swim in tributaries