Ah, I had forgotten about the Walther boats (such a thing figured in Neal Stephenson's Cryptonomicon, IIRC). I was thinking more something like the Type XXI, with better hydrodynamics for increased submerged speed, longer endurance to reduce reliance on the 'milch cows'. Add in snorkels, burst radios and doctrine less reliant on constant chatter, maybe homing torpedoes, and they might have a better chance of surviving? OTOH, the requirement to keep your head down much of the time, reduced radio traffic, and allied air cover chasing away the Condors means those subs, even if they survive, will have a much harder time finding targets to sink.
As for peroxide, that's an interesting topic. At high concentrations it's quite spicy stuff, much different from the diluted solutions you might have used to blond your hair in the folly of your youth. It was also studied quite a lot post-war for use as oxidizer in rockets (John Clark's "Ignition!" has a chapter on it. As an aside, it's a very entertaining book, highly recommended! I think it's out of print, but you can find a PDF online.) . That super-toxic fuel/oxidizer combos like hydrazine and nitrogen tetroxide eventually won out is testament to the difficulties encountered with peroxide. The problem is that the decomposition of peroxide into steam and oxygen is exothermic, and even minute amounts of impurities in a storage tank can catalyze this reaction. By the time you hear a sharp PING from you peroxide tank and/or notice a pressure rise on the gauge, it's time to leg it. Oh, I forgot, you're on a submarine, there's nowhere to run. See also the 2000 Kursk submarine disaster, most likely caused by a torpedo using peroxide as the oxidizer.
