How much more difficult would the U-boat war have been without breaking Enigma?
Short answerHarder and less efficient, with more escorts needed, but HF/DF and traffic analysis would still provide warning and the final outcome might not change.
NotesCodebreaking was one part of the wider signals-intelligence system used against U-boats. HF/DF could locate a transmitting submarine, while traffic volume indicated that a wolf pack might be assembling even when the message remained unreadable. Enigma supplied the orders and details that made this warning process faster and more precise. Without the decrypts, convoys would likely have required more escorts and less efficient routing. Losing Enigma would make the campaign harder, though it would not automatically reverse the outcome.