Eric Portman's character uses the captured German flyer's uniform, including 'flying boots', when going ashore to find fuel, but when the German flyers are first taken they all swim onto the sub barefooted.
When the sub returns to base Taylor (John Mills) in light coloured jacket, trousers, and white wind jammer type jacket, climbs the gangway ladder to report to the c.o.. There's then shots of the crew coming ashore then one of Taylor, now in naval uniform,with the co watching a sub leave.
The Feldwebel has a German uniform, but British signs of his grade on the arms. A German NCO has signs on the shoulder, not on the arms, and not this kind of sign.
The sub is submerged; the Captain is using the periscope to view a target. He then orders a 15 degree turn, but the Captain continues to view the target without moving the periscope to adjust for the turn.
The Germans do not notice Hobson's Yorkshire accent.