it seems that you would remove the need to have a pump to build pressure, but you would still need a pump to move the product water ( perhaps creating the negat

it seems that you would remove the need to have a pump to build pressure, but you would still need a pump to move the product water ( perhaps creating the negative pressure on the outflow side of the membrane in the same step ) Way less energy, apart from the challenges of operating at depth. There would be power delivery constraints, and the basics of plumbing.