fruitless effort; wasting time on an impossible task
A yojijukugo literally meaning 'scooping up the moon from the bottom of the sea'. Used to describe a completely futile endeavor, akin to 'tilting at windmills'.
His plan is like trying to scoop the moon from the sea.
無駄骨折り is a more common, everyday expression for wasted effort, while 海底撈月 is a literary yojijukugo with a poetic, impossible-task nuance.
A yojijukugo of Chinese origin, literally 'sea-bottom scoop-moon'. The phrase evokes the impossibility of scooping the moon's reflection from the water.