I found this through the Google Images search:
Direct link to this is http://www.uscg.mil/hq/g-cp/history/gifs/U_BOAT_1.jpg
While it is both B&W and too contrasty, it seems pretty clear that on this boat there are no large rust stains on the top section of the hull. There is some sort of discoloration near the water line, but I couldn't venture to say from what.
On the other hand, the following image shows about as much as you could want to know about both above and below-waterline weathering:
Direct link to full size version of this is http://www.navyphotos.co.uk/uboat%20u1181b.jpg
Hope this helps.