Water in open ocean is not still - there are currents. I suppose if you had a large enough operation you could create a gradient of sufficient size to be self-

Water in open ocean is not still - there are currents. I suppose if you had a large enough operation you could create a gradient of sufficient size to be self-sustaining without further mechanical agitation (such as your example of towing behind a ship). It would be an interesting exercise to see a large collection of these in operation. Heck even if you had to pump ocean water on land and back out to dilute the brine this process would still use significantly less energy overall than current desalinization plants. Speaking of current plants, a recently constructed one in Carlsbad CA leverages the cooling outflow of a power plant to handle the dilution. The power plant was already there and so was the outflow - these things can be planned to leverage overlapping uses to further increase efficiency.