Both were the successful leaders that their era and countries needed.
Donitz almost defeated Britain using his submarine fleet alone (he just didn't get the number requested.)
Rickover's accomplishment wasn't demonstrating that nuclear power was useful, it was getting the support and funding to actually roll out the new technology. To this day, no other country has a nuclear aircraft carrier besides the US and France.
75% of U-boat crew died in combat, primarily because Donitz never changed their cryptography and gave them tons of radio traffic to decipher.
"Radio traffic compromised his ciphers by giving the Allies more messages to work with. Furthermore, replies from the boats enabled the Allies to use direction finding (HF/DF, called "Huff-Duff") to locate a U-boat using its radio, track it and attack it.[64][65] The over-centralised command structure of BdU and its insistence on micro-managing every aspect of U-boat operations with endless signals provided the Allied navies with enormous intelligence"
They did change their cryptography. They changed the settings on the enigma machine daily, and they also went through various versions through the war.
Changing the settings wasn't enough after the enigma machine was cracked. And more to the point, Donitz management style made their radio traffic easier to decipher, and he was extremely slow to react when it was clear crews were dying because their radio traffic wasn't secure.
https://en.wikipedia.org/wiki/Karl_D%C3%B6nitz
Nuclear power is a game-changer, allowing aircraft carriers to sail completely around the globe at speeds fast enough to barefoot water-ski behind.