[sikonomial]

The container can be similar to a hydration bladder (Google what it looks like) that is slightly more dense than salt water.

1. At bottom you fill it with fresh water

2. It floats to the surface

3. At the surface you just empty it and remove the fresh water

4. It starts sinking

5. Jump to step 1

[mattmaroon]

Right. You’re glossing over the energy required to fill a bladder at sea depth with enormous pressure on it. That requires a pump and a lot of power, just like pumping it to the top does.