Potential density
Potential density is a dynamically important property: for static stability potential density must decrease upward. If it doesn't, a fluid parcel displaced upward finds itself lighter than its neighbors, and continues to move upward; similarly, a fluid parcel displaced downward would be heavier than its neighbors. This is true even if the density of the fluid decreases upward. In stable conditions (potential density decreasing upward) motion along surfaces of constant potential density (isopycnals) is energetically favored over flow across these surfaces (diapycnal flow), so most of the motion within a 3-D geophysical fluid takes place along these 2-D surfaces.
Cavitation modelling
If the pressure is greater than vapor pressure, then the fluid is liquid, otherwise vapor. This means density of liquid water is considered as the density of fluid if the pressure is greater than vapor pressure and the density of water vapor is considered when pressure is less than vapor pressure of water at the ambient temperature.