What do you mean like a pool of water?

There is "inner tranquility" in ancient poetry, which is also an idiom, meaning: to describe a cool head without distractions.

"Heart-like water" should refer to calm water, and "heart-like water" is also used to describe calm mind, no distractions, and no waves like calm water.

The origin of "inner tranquility": Tang Juyi's "Sacrifice to Assistant Minister Li": "The world is different; Teeth are rolling and waves are everywhere. Who is the public, quiet inside; The wind and rain are like rain, and the chickens are like singing. "