like water

Do you think Aiko's won the lottery or something? She's been spending money like water lately.

Well done! If you spend money 'like water' it means you spend it freely without control.

I try to let creativity just run like water when I'm writing songs.
曲を書いているときには想像力のおもむくままにまかせようとする。

I want to be able to spend money like water
湯水のようにお金を使えるよう［身分］になりたいと思う

It's like water off a duck's back.
そんなことしたって何の効果もない