It is obvious; Rovo said, it was not necessary, etc. - פשיטא אמר רבא לא נצרכא כולי
In the הו"א we may be either discussing two ships belonging to the same person, or we may be discussing ships that belong to different people with an עירוב and it is easily understood why if they are not tied it is forbidden לטלטל מזו לזו.
He tied it with something that holds it; it brings Toom’oh to it - קשרה בדבר המעמידה מביא לה טומאה
According to רש"י we are discussing that the ship becomes an אב הטומאה by touching the iron chain; however according to תוספות we wish to make the ship into a stable (non-moving) אהל; the material of the chain is irrelevant.