An idiom meaning someone who is in their element—vibrant and energetic after returning to an ideal environment (literally, a fish that has found water).
大学に戻った彼はまるで水を得た魚のように生き生きしている。
Back at university, he is lively as if a fish that has found water.
新しいチームで本領を発揮して、彼女は水を得た魚のようだ。
Showing her true abilities on the new team, she seems in her element.
都会に出てきて初めは苦労したが、今では水を得た魚のように楽しんでいる。
She struggled at first after moving to the city, but now she's enjoying herself as if in her element.