Quite possibly it is a code marker. No one is going to the effort to dump a stripped chassis in the water for no reason other than laughs ( unless it was Americans dumping stuff in the water after the war ). We know, if it is sale able scrap, it gets scrapped or used for something. So it was dumped in the middle of no where for some reason. Humans don't expend energy doing nothing, unless it is politics or art work ( ha ).
Z