The decision is yours...but consider something...rust occurs when bare metal is exposed to air...in the case of a U-boat, although they spent most of their time on the surface, those surfaces were almost always wet, even when they were on the surface is good weather. Although oxidation does continue to happen underwater (at a considerably slower rate), I would tone down the rust. What you tend to see on U-boats is more of a light streaking from the salt deposits mixed with a subtle rust.
A good example would be to look at heavy equipment...bulldozers, dumptrucks, any sort of machinery that's left outside to the elements and has areas of worn paint.
My belief is that if you do too little, it's still believable, because even the dirtiest of vehicles were clean at one point in their lives. If you go overboard (no pun intended), that's what people will notice and remember about the model.
That being said, you do what you think looks good!
Jeff