Headgear
The Schirmmütze
The Schirmmütze or visor cap was the regulation service dress headgear of the Kriegsmarine officer. After the outbreak of war, the blue top cap was regularly worn at all times in the year by all officer ranks. The wear of a white top cap by ships captains, especially in the U-Bootwaffe was an unofficial fashion to be worn only at sea and officers were expected to revert to the blue top cap one back ashore, This of course was frequently ignored.
Original Kriegsmarine Admirals Cap. ( Courtesy F. Saez)

The highest grade cap which would normally be found being worn by a U-Boat Officer was that for the ranks from Korvettenkapitän to Kapitän zur See, witha broad band of gold embroidered oakleaves on the peak.
The most commonly held rank by U-Boat commanders was Oberleutnant zur See or Kapitänleutnant zur See. The hat for these ranks featured a row of scalloped gold bullion embroidery around the edge of the peak.
Warrant Officer ranks wore a similar visor cap but with a plain shiny black peak the edge of which is bound with leather.
The Bordmütze
The Bordmütze or boarding cap was worn by officer ranks with a woven gold piping around the edge of the flap. It was common for officer caps to feature enlisted quality insignia as here, though officer quality insignia woven in gold wire did exist.
Sailors Cap (Matrosenmütze)

The sailors cap tended to be worn by U-Boat men only when on shore, its long ribbon tails being hazardous in the close confines of a submarine. The Bordmütze was most common worn on board ship by crewmen.