About rust: yes, U-boats did pick it up on patrols (and of course they didn't have chipping, scraping and paintings parties), but I think you ought to go lightly on it. I've seen a lot of fine models of U-boats made by much better modelers than I am which i. m. h. o. were ruined by the heavy-handed addition of too much rust. Remember that there are other kinds of weathering, too: for ex., drybrushing with a lighter color over a darker base paint (or vice versa) to represent dirt and grunge (especially useful for "popping out" rivets). For the underwater part of the hull you can also have algae and barnacles. So it's possible to have a reasonably "used" looking boat without turning it into a real rustbucket.