The American navy tested Erich Topps U-2513, type XXI(Electro boat) crush depth and their results were 900ft. The Brits tested another type XXI and the hull collapsed at 800feet(Less then the conventional U-Boat). Like someone said earlier, it depended on how well made they were and they were all different from each others. All U-Boats were what we would call today "handmade", only the later type XXI was factory line assembled, in a module assembling system, like manufacturing is today. The later U-Boats were hastily assembled and usually of poorer quality, because of all the strains and bombings on German factories late in the war.