The way the command diving (and surfacing) is repeated by everybody is to sharpen everybodys mind for the status of the boat being submerged now, or not; also to not have anybody missing it. Its not good if somebody tries to open the hatches while the boat is in diving mode.
For the same reason, the German numbers 2 ("Zwei") and 3 ("Drei") are not being pronounced that way, since the words sound almost the same way in German, both end - by sound - in "aye". In certain context, when you want to be sure the right number gets understood, you would then say "Zwo" (2) instead of "Zwei". Again, purpose is to sharpen the receiver's awareness of the correct info.
Its also a tradition, kind of, a ritual to strengthen team spirit, like the beers at the end.