I can imagine a dock that has the ability to load extra temporary mass on and off a cargo submersible, all loading done completely underwater, to offset materia

I can imagine a dock that has the ability to load extra temporary mass on and off a cargo submersible, all loading done completely underwater, to offset material being taken off the submersible. If the efficiency increases were significant, this could become worthwhile. However it's likely hard to beat the efficiency of how many cargo containers you can stack on a normal cargo ship, particularly while fuel is cheap.