水の心 joins 水, water, and 心, the heart, with の between them. The の keeps it clear of 水心, a swimmer's feel for the water. The phrase holds more than one water, and every one of them is live. A still heart mirrors whatever comes, the calm that karate trains for. A flowing heart takes the shape of any vessel. Which water you see in it is yours.
The image is older than the phrase: a 万葉集 song likens a clear heart to cold water, the phrase itself is in a love poem of the 古今集 of 905 whose water's heart runs swift, a poem of 1151 gave the dictionaries their headword 心の水, the heart likened to water, and the words have since carried the 荘子's still mirror, the formless water of the 老子 and Musashi, and 高田敏子's poem 水のこころ of 1975, set in fifth-grade readers from 2001.
We want you to be as happy with the meaning as you are with Master Takase's art. To learn more, visit Philology of 水の心.