As the commander of Germany's submarine forces both before and during World War II, Commander-in-Chief of the German Navy from 1943 until the end of the war, and Hitler's successor as head of state, Grand Admiral Karl Dönitz Memoirs offer a unique perspective on the inner workings of the Kriegsmarine.
Granted, there are some areas where this autobiography is necessarily a bit self-serving. Dönitz generally shows himself in a favourable light, though this is hardly to be unexpected. And there are some places where his opinions, despite being able to use the Kriegsmarine war diary for reference, prove to be wrong, such as the importance of Allied code breaking, which he believed to be of negligible influence on events. In his defense, Dönitz died before the achievements at Bletchley Park were declassified, so if he was unaware of what had actually happened, so was everyone else outside that group. He gave his best estimate of the situation based on the information available to him.
The question of just how committed Dönitz was to naziism is never entirely answered, though he does take care to argue that, whatever his commitment may have been to the party's political goals, he disagreed with its racial policies. He suggests that naval and military support for the nazis was mostly a matter of expediency. Dönitz argues that the nazis and communists were both viewed as potential participants in a civil war. Since the military lacked the capability to fight both should that occur, they backed the nazis as the lesser of two evils. He attributes much of his own support for the regime, once it came to power, to his traditional Prussian upbringing, with its emphasis on duty and tradition of military obedience to the civil government.
Dönitz claims he was not informed of what was going on in the concentration camps until after Hitler's suicide. Before that, he says, he was too busy running the submarine arm, and then the Navy itself, to concern himself with what was going on in the conquered territories. Since the nazi regime kept the implementation of the "final solution" tightly compartmentalised, it's entirely possible he was correct.
It's interesting to note that his few known actions directly relating to the Holocaust indicated disapproval. He was a co-signator, with Admiral Lütjens, of a letter objecting to the persecution of German Jews. And one of his first actions as head of the German state was to fire Himmler and give orders to investigate what had happened with a view to prosecuting the perpetrators. As he was, himself, arrested by the Allies a few days later there's little point in speculating what the ultimate outcome might have been.
In his Memoirs, Dönitz lays much of the blame for Germany's failure to develop advanced u-boats until too late to affect the war to Hitler's interference. He also echoes the well-known Kriegsmarine complaint that the war began much too soon—long before they were ready to fight it properly. (The German Navy was preparing for a war that would start about 1945.) Instead of having a fleet of 200 u-boats, which Dönitz considered necessary at the beginning of the war, he had only 57, many of them too short-ranged for use outside the North Sea. Where he had planned to keep 60 to 80 boats in the patrol area in the Atlantic, there were times when no more than two or three were actually available.
For anyone interested in the Battle of the Atlantic, this book is primary source material, and highly recommended.