noun
useless book learning; theory without practice
Idiomatic and often jocular; literally 'swimming practice on a tatami mat', implying knowledge that cannot be applied in real situations.
いくらマニュアルを読んでも、畳の上の水練だよ。
No matter how much you read the manual, it's just useless book learning.
His knowledge is all theory and no practice; it's useless in real work.