[sikonomial]

You don't need to pump up the water. Fresh water is less dense than salt water so it will float up to the surface on its own.

[patall]

That would be a perpetuum mobile. You either have a pressure difference at the membrane or between outside and inside the tube.

[sikonomial]

The process would be like this:

1. Take in salt water

2. Spend some energy to separate salt from water.

3. Put fresh water into a container.

4. The container containing fresh water will raise to the surface, since it is less dense than salt water.

There is no perpetual motion.