German U-boats had torpedoes, diesel, and oil, and that's it. The lack of any kind of amenities for the crew would beggar the imagination. The movie "Das Boot" can give you at least a rudimentary idea of how bad the living conditions were on board. But they definitely didn't go around with tons of treasure on board.

Later in the war, the German's built supply submarines called 'milk cows' that would go around re-supplying bases and ships. But the treasure on board was usually fresh and canned foods, crates of apple juice (one of the few treats the submariners got to enjoy), and spare parts. But that's it.