Yes, it's the fresh water that's rare, and becomes salty in the process so it's sort of like stored solar energy from rain (mostly from the oceans)... osmotic power.
The power density even at 40% efficiency is pretty high (300Wh/m3) so major rivers (Mississippi 20,000m3/sec or 72Mm3/hr) would generate 20GW fully exploited.
The problem has always been the growth of biofilms on the surfaces of the membrane, which break them down and/or lower their efficiency.
https://en.wikipedia.org/wiki/Osmotic_power
The power density even at 40% efficiency is pretty high (300Wh/m3) so major rivers (Mississippi 20,000m3/sec or 72Mm3/hr) would generate 20GW fully exploited.
The problem has always been the growth of biofilms on the surfaces of the membrane, which break them down and/or lower their efficiency.