[brilee]

Desalinated water is also less dense than normal seawater, so the water column inside the output pipe would create a pressure imbalance with the water column outside the pipe, assisting in the outflow? I'm having trouble figuring out how to resolve this seeming perpetual motion machine

[VonTum]

The minimum pressure differential needed to perform reverse osmosis is bigger than the pressure differential between a collumn of fresh and a collumn of salt water of equal height.

[josefx]

I think it would stop in an isolated setup once most of the water is desalinated.

[shiandow]

That still makes no sense, water can't desalinate itself in the same way it cannot spontaneously cool itself.