The mebrane desalination idea is great! It provides a low tech, minimal moving-part solution to an incredibly important problem. We can kick it up a notch by running it both ways... Build several under water tanks near the plant. The top of the tank contains a turbine for electric power generation and there are ports near the top to seal or open the tank to the outside water. On the bottom is another membrane, just like the one that did the topside desalination. Here is the cycle: 1) Pump water into the processing plant on land. Gravity pulls fresh water through the membrane. Send the salty side back through a tube into the bottom of one of the tanks. Because the super salty water is heavier than the surrounding sea water, it goes to the bottom and normal sea water escapes out the ports. 2) After a predetermined amount of super salty water enters the tank, the ports are sealed. The only way water can leave now is through the turbine. The salty side is now sent into the next tank. 3) Osmosis takes over in the first tank as fresh water flows through the membrane into the super salty water. This is a very forceful process and would increase the volume in the tank pushing water through the turbine. The process would be allowed to continue until the salt differential was too low to produce useful turbine output. 4) The next tank would then be sealed and start producing power.
The only moving parts are the turbines, the ports, and the salty side tank selection manifold. This process could regenerate some of the energy for the pumps. You would not recover any of the energy if you just returned the salty side into the open sea.