You're not really displacing all that much mass though surely? The column is surrounded by water on all sides, i.e. you're removing relatively little mass from the top of the tube, and the entire ocean is pressing in on the rest of it.
Not quite. You need a pressure difference at the bottom. The fresh water will come from reverse osmosis at lower pressure than the ocean at that depth. And it will be not just little lower pressure, but pressure created by ~500km of water, because if not this then why to go to so deep?
Right, which would be provided by the pipe walls / container membrane resisting ocean pressure, and then reducing the weight of fresh water pressing down on the top (by removing it via pumping off the top).
