Hello, Gumby!
I'm familiar with Patterson's book about the Suhren's U-564. And there author clearly states that only IWO, IIWO and Navigator (Oberstuermann) were acting as WO, while bosun was indeed "number one of the crew".
Then there is for example Stern, who claims in his book "
Type VII U-boats" that also bosun stood watches as WO, i.e. he would have been the fourth WO on board. However, I still doubt this. Or have you ever heard about US Navy CPO or Warrant Officer, who has driven a Fleet Boat on patrol or elsewhere? If I'm wrong, just get me straight.
What I'm thinking about is PO or CPO acting as WO while boat/ship is tied in the pier. As far as I know it's done all the time even nowadays, so why not 60 or 70 years ago, when many things were much more simple? So, maybe Stern got these two situations (enroute/in pier) messed?
Another possible answer to the question might be also that Bosuns striking on their navigator's qualification were given WO duties. But, as far as I know, Kriegsmarine was pretty much "no papers - no job" employer, i.e. if you did not have a proper qualification there was less chance you got the job. And I have a feeling that this was one big difference between, let's say, Kriegsmarine and US Navy, for example. Someone from Germany might be able to confirm this, I believe.
-RC-