OTOH, just because you are resting on the bottom it does not mean you are undetectable. I would suspect that a WW2 sonar might have a more difficult time discriminating the submarine from the sea floor and you can pass yourself off for a wreck but that's it. For what is worth I found this
reference
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Schnorchel U-boat has been the chief concern of surface craft, whose main difficulty has been that of distinguishing between a bottomed U-boat and a wreck. An extensive survey of wrecks in British waters helped considerably in solving this problem.
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As I recall the Lusitania wreck was used for ASW target practice and is said to be littered with hedgehog projectiles. At one point one was brought up by divers who had exchanged it for a bottle, needless to say it was quite a shock
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that's not how sonar works 
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Actually different materials do have different reflection characteristics, though how well ww2 vintage sonars could exploit that I would not know.