"Mass Fraction of Water" is one of a number of $\textit{Concentration}$ quantities defined by ISO 80000.
$w_{H_2o} = \frac{u}{1+u}$, where $u$ is mass ratio of water to dry water.
Mass Fraction of Water is one of a number of Concentration quantities defined by ISO 80000.
Applicable units are those of quantitykind:MassFractionOfWater
Mass Fraction of Water