Like a fish in water
For someone to be in an environment or with a person that best suits them
Positive
To be in one's element
None
This story is from the Romance of the Three Kingdoms, one of the great works of Chinese literature. Lots of details are left out for this chengyu explanation, but if you want to know the rest, you can check out the original source text or listen to John Zhu's Romance of the Three Kingdoms podcast
《三国志》(Sān Guó Zhì) by 陈寿 (Chén Shòu). The historical text covers the events of the end of the Eastern Hàn Dynasty (东汉, Dōng Hàn, 25 - 220) and the Three Kingdoms period (220 - 280)
At the chaotic end of the Eastern Hàn Dynasty (东汉, 25 - 220), the warlord 刘备 (Liú Bèi) was one of many great men warring for power. In order to realize his ambition of unifying the country, he recruited as many talented and intelligent men as he could, including the great strategist 诸葛亮 (Zhū Gě Liàng). Recruiting 诸葛亮 had been an extremely challenging feat. The strategist was a recluse who lived a ways away from civilization, and it took 刘备 three long and grueling trips to his cabin before the two were even able to meet. When they finally did meet, though, 刘备 told 诸葛亮 of his aspirations, and and 诸葛亮 responded with suggestions and guidance as to how to unify the kingdom and defeat the two other major warlords, 曹操 (Cáo Cāo) and 孙权 (Sūn Quán). This delighted 刘备, and he managed to convince the recluse to become his military advisor.
诸葛亮 did his utmost to aid 刘备, and 刘备 deeply trusted and respected him and his decisions. However, in the beginning, 刘备's unflinching trust in a man that he'd only just met grated on the nerves of his two generals and sworn brothers, 关羽 (Guān Yǔ) and 张飞 (Zhāng Fēi), and they often voiced their displeasure and concerns that trusting the scholar would end in ruin. 刘备 tried to pacify them by explaining that 诸葛亮 would help him realize his great aspirations, adding that "for me, getting the services of 诸葛亮 is like a fish getting water."