[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.

[patall]

Then you could also do it at the surface. But they do it a depth because they want a pressure difference on the two sides of the osmosis membrane. You somehow need to generate that pressure difference and the energy you need for that is minimum equal to the amount you need to move the freshwater.

Oh, and you will have to do it continuously, not with a 'container'. Existing desalination plants produce hundreds of thousands of cubic meters of fresh water per day.

[XorNot]

You pump water off the top of the pipe, reducing mass and pressure at the bottom and thus allowing for desalination. It's a classic distance x force trade off: it's easier to use a static membrane, and a low pressure pump then build a high pressure pump at the surface.

Nothing in this system is 100% efficient, so how you organize your components can make a huge difference.

[mattmaroon]

You have had to spend energy to get the floating container to the bottom.

If you filled it with something heavier than water, or left it open to the elements to sink, you still would have to spend a bunch of energy to pump it clean at the bottom.

Probably still easier to just pump the water up.

[sikonomial]

The container doesn't have to float. The container could have density of 1020.00001kg per m3 and it will sink. Saltwater is 1020kg per m3

Then when you fill container with fresh water 1000kg per m3 it will float.