Go easy on the weathering. U Boats did mess up if out on long patrols but no rust from the flooding holes. Areas such as the anchor, winches, tie up, hand rails joins and bases etc.are more appropiate.

Look at photos of the U Boat towers and follow the rust areas. There are excellent models shown on the net showing very good rusting and discharge holes oily dark discoloring BUT as in all weathering apply the old KISS principle, Keep It Simple Stupid, I live by this all the time. Do not overdo weathering, study photos.

Re aircraft.

I have seen lots of weathering on aircraft models that leave me wondering about their maintenance, I flew in a variety of aircraft over the years and none of them were in the 'weathered' condition you often see. They were well maintained and always clean.