Permeability is used in ship survivability and damaged stability calculations in ship design.
In this case, the permeability of a space is a percentage from 0 to 100.
Alternately, the permeability may be a coefficient from 0 to 1.
The remaining volume [not filled with seawater] being occupied by machinery, cargo, accommodation spaces, etc.
Typical values from the International Bulk Chemical Code are: This implies that for damaged stability calculation purposes, machinery spaces are only 15% full with machinery by volume (100% - 85% = 15%).