Why a wartime U-boat pen was a hard sabotage target

If the panel reached a German U-boat pen through a portal to 1942, what would they take?

An Enigma machine is one suggested souvenir, but the base's security makes getting shot the immediate objection.

An Enigma machine is an appealing souvenir in the fantasy, alongside small machinery components. Survival is the immediate problem because an operating U-boat pen would be heavily guarded. The security argument distinguishes forced labour in construction and factories from access to submarines being loaded with live ammunition. A counterexample raises the use of forced labour around aircraft, prompting a distinction between the loss of an individual aircraft and the greater strategic cost of losing a submarine. U-boats are especially important in this comparison as one of Germany's remaining means of striking beyond enemy lines later in the war.