spend money like water 挥金如土;花钱如流水;花钱大手大脚;花钱如水;挥金如水

【例句】
Sailors in shore leave often spend money like water.
获准离船上岸的水手往往花钱如流水。

It is better to be frugal; don't spend money like water.
最好还是节俭些，不要乱花钱。