> all knew that the Enigma had been cracked, but Admiral Doenitz refused to believe it.

Whereas:

  The dropping results made Admiral Dönitz suspicious. Although reassured by the Abwehr, the German Foreign Intelligence, that Enigma was unbreakable, he insisted on improving the security of Enigma. On 1 February 1942 the famous Enigma M4 model with four rotors and new key sheets were introduced. 

~ https://www.ciphermachinesandcryptology.com/en/enigmauboats....

~ https://uboat.net/technical/enigma_ciphers.htm

There were multiple Enigma variations, based on rotor choice pool sizes, number of fittable rotors, time cycles to changing procedures, etc. Some naval enigma variations were broken, others weren't.