Hmm, I thought that I had replied to this, but I must have forgotten (or perhaps it failed to post). This is an interesting idea, but the difficulty is that there are no floating transshipment docks in reality, so this would seem weird and counter-intuitive to the player. Better, I think, would be an import dock (shoreline industry) which simply generates large amounts of coal, and where the boats coming to and from it are simply imaginary. That would, at least, be more consistent, I think.