This method of generating power was ascribed to Prof. Sidney Loeb around 1973, then working at the Ben-Gurion University of the Negev, Beersheva, Israel.
Pressure retarded osmosis (PRO)uses the salinity gradient energy retrieved from the difference in the salt concentration between seawater and river water which corresponds to a hydraulic head of 270 metres, which is equivalent to pressure of 26 bars.
The optimal working pressure is only about half of this, around11 to 15 bars The picture below shows a submarine hydroelectric power plant anchored to the sea floor.
Fresh water, with potential energy (a head of water) of about 100 metres is piped downwards to a submerged hydraulic turbine which drives a conventional electrical generator.
The high pressure fresh water is discharged into a low pressure tank maintained at the lower submerged level.
The low pressure is maintained (counter intuitively) by the fresh water diluting the external salt water through an osmosis membrane, so that it is essentially 'sucked out' of the tank by seawater.
That is to say that the fresh water diffuses out in the sea by osmosis, through a barrier of semi-permeable membranes, which prevent the passage of brine (salt-water) into the tank but allows fresh water to flow outwards
In practice the fresh water will generally contain dissolved salts and particles such as sand, silt and agricultural residues. A flushing mechanism would probably be required to prevent accumulation obstructive deposits on the fresh water (low pressure) side of the membranes. Obstructions on the sea surface of the membrane would tend to be flushed by the escaping water.