The extra pressure generated by the osmotic force of the fresh water coming into the salt water at the lower chemical potential is given by:

The max work will occur if we expand the piston reversibly. Thus the external force will also always be 7.09 atm ( in addition to the 1 atm that was already there on both sides due to the atmosphere but cannot be used to drive a generator). Thus the max work per liter of fresh water coming in is just

Note that the 1 atm on both sides is just the atmosphere pushing against itself and I cannot use that pressure to drive my motor so I did not include it.