An idiom meaning a small or too-late effort that has little or no effect—'a drop in the bucket' or 'too little, too late.'
あの会社への小さな投資は焼け石に水だ。もっと大きな支援が必要だ。
A small investment in that company is a drop in the bucket; it needs much bigger support.
一回だけの注意では焼け石に水だから、継続して指導しよう。
A single warning won't do much—it's too little, too late—so let's continue to provide guidance.
彼の借金が膨らんでいる中での少額返済は焼け石に水に過ぎない。
In the face of his mounting debt, a small repayment is just a drop in the bucket.