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.

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.