Hi Arjan, 
I'm not sure, but I believe the blue hat top has to be worn from all KMsoldiers  
in wartime . Only the Commander of the boat or ship had the white top ...  
in wartime.
It was similar during my time with the Bundesmarine; all ranks had the white top, but we all received the dark blue one for the case of war and the hat's band - in peacetime showing the ship or the unit - has to be changed to "Bundesmarine" for secrecy reasons.
So, on wartime pics you can only find dark blue hat tops and bands with "Kriegsmarine"-lettering.
:-H 
Michael
PS.: ... I have a pic, the master of signals was wearing a piece of cloth on his head - like a  

 . But this was at the time Zerstörer Bayern was sailing under the Jolly Roger when all NATO-ships were leaving SNFL for X-mas! Long time ago  

  in a cold war.... but fortunately allways with white tops... oh, upps  
 
 Good night  
 
 PPS: Arjan, on the pic with the mines, there's a can in the forground - between the left an right row of mines. And you can see an equal can on the other side of the boat. Perhaps for smoke - you remember the fittings, we puzzled over some days ago? For the french and the german smoke buoys ...