The significance of the U-boat Enigma machines has been lost in many historical accounts of WWII today.

In August of 1941, a U-boat, U-571 (I could be wrong about this number - someone might want to double-check that), was captured - along with its Enigma machine. At that time, all Enigma machines were using three "rotors" or settings wheels to perform their encrypt/decrypt.

The head of the Nazi navy, Doenitz, suspected that the Enigma on U-571 had been captured, and he asked for four-rotor Enigma machines to be made specifically for the Nazi navy. They were, and these were introduced in  February, 1942. For the next nine months, the German navy could operate with much more success than it would have if they had retained the three-rotor machines. The methodology of cracking the four rotor code was largely the same as the three rotor (army and general communications) Enigma machine, but the combinatorics required a huge increase in resources to decrypt a four rotor machine's messages.

It was captured U-boats with intact four rotor machines that closed the door on the Nazi navy's communication security and gave the allies a huge leg up in operational intel.