Fresh water (non-saline water); water without salt — i.e., potable or freshwater as opposed to seawater.
台風で井戸が濁ったが、しばらくすると真水が戻った。
The well was cloudy after the typhoon, but fresh water returned after a while.
船は真水のタンクを満たして出港した。
The ship left port with its fresh-water tanks filled.
In economics, 'true fresh money' — funds used by the government for fiscal stimulus (i.e., money that actually flows into the economy).
政府は景気対策として真水の支出を増やした。
The government increased real fiscal spending as an economic measure.