Hello MPC and all!
I am agreed that the Party membership doesn't mean anything.
Some people joined the Party (and I have very bad opinion about the Party, just to explain) because situation after WW1 was so terrible and hopeless that the Party and its leader looked like the light on the end of a tunnel...
So, in 1933 the Reichmarine, and very soon the Kriegmarine had different kind of people, monarchists, democrats (all kind of, from social democrats to democratic Christians), non-political, and Nazis.
After all, it is natural to some military people to prefer "stronger" state, and the Weimar Republik looked very weak to them- and not just to them but to many other people.
Not all of them were "born" Nazis, and most of them overlooked things like racial program of the Party, for example, and just wished stronger state and the Armed Forces etc etc.
It is usual that people see just things they like, and not the other things.
Once the Party came on power, it was too late.
But, the bigger problem was that younger Germans, those who were ten years old in 1933 (for example) passed through strong "brain wash" in their youth, and during the war they were in the Armed Forces, and they were influenced with strong Nazi ideas. They weren't guilty for that,
propaganda and "new" school system weren't just in Nazi Germany but also in all non-democratic systems in history, especially after WW2...
It is clear from the interrogation reports of captured German submarines, that the younger were influenced with Nazi ideas. Interesting, in the same time they had very low general knowledge- seems that it was connected...

It is not important how many U-boat commanders were nazis, or the Party members. After all, where was the boundary between political loyalty, or opinion, and the pure patriotism. Bad country, good country, my country...
What the most important is that we understood that such regimes are very dangerous, not just for the other countries, but to their own countries.
I watched on TV some one former commander of one Kriegsmarine torpedo boat in the Adriatic in 1945 , he was very young during the war, and he said that he had fought for his country, he was proud those days because of his role, but after the war he realized that the idea was wrong. He didn't blame the enemies, but his own leadership which involved him and the others in war for the wrong ideas.
And, I am happy because he said so!

Best regards!
Funkmeister