Once again I'll say it: removing scuba unit, swimminbg away, swimming back and replacing it, is NOT in the course. That is just made-up by some instructor, or a poster,
Why would someone ever leave their gas supply if it's still working? I would be teaching you to breathe from the BCD inflator and tank valve if your regs were fubar before ever leaving your gas source. Only if it were totally hung up on something like spider wire and you couldn't cut it (perhaps dropped tool) would there be a reason to swim up. Swimming away and back? Let's see ... you are caught in wire. You swim out of your rig to a nearby shipwreck and use a rock to break a porthole. You enter, swim inside 3 decks into the engine room, where you find some wire cutters and return to your rig ... kind of thin ... anorexic. Which is why you fit through the porthole.