The fresh water allowance is for use when the vessel is loading in fresh water, which does not have the same buoyancy as sea water (assumed to have a density of 1,025). So, if loaded down to F in fresh water, the vessel would rise to S by the time it reaches the open sea. The person responsible for loading the vessel, therefore, must know the density of the water at the loading berth.
Dry Bulk Shipping News - Shipbroker Notes