A drop of water thrown on a red-hot stone is of no consequence. Instead of cooling the stone, the drop of water evaporates in an instant. Yakeishi ni mizu expresses a grossly inadequate remedy to a problem.
A: Kongetsu wa monosugoi akaji na no yo. Sukoshi shokuhi o setsuyaku shite mita kedo, yakeishi ni mizu de doonimo naranai wa.
A: We are ‘way in the red this month. I tried to spend less for groceries, but it’s a drop in the bucket. I just don’t know what to do.