This is a reversible process, as you can get the energy back by using the osmotic pressure to drive a piston. A desalination process that used a lot less energy would let you build a perpetual motion machine, and is seriously against the laws of physics.
What if we use the oceans water pressure in our benefit?
For example, deploy this membrane 100m under the ocean, and have a pipe from the membrane going into a tank on the beach nearby.
As we pump fresh water out of the tank on the beach, the oceans own water pressure, pushes water through the membrane and up into the tank on the beach. That way the pump on the beach requires “very little” energy to move the water horizontally on the surface.
The excess brine build up would still be an issue but that’s always an issue.
Unfortunately, that's not how pressure works. There's a 100m column of water above the salt side of the membrane, but there's also a 100m column of water in the pipe on the fresh side. Those columns generate an enormous pressure, but it's pushing from both sides, which doesn't help you get water through the membrane.
So the pumps still need energy by “very little” energy because they are doing relatively little work.
There is no free lunch - for the membrane to experience 100m of water pressure on one side only, something else has to be reducing the pressure by the same amount on the other side.
In that scenario, why do you think it helps to put the membrane 100m deep, instead of 5m deep?