Joachim Donitz was Werner Donitz's grand-uncle. He was the second-youngest of the four.
Joachim and his brothers (Rudolf, Walter, and Friedrich) fought in separate branches of the Nazi army in WWII. Joachim fought in the Kriegsmarine and died on the Bismark when it sunk.
Werner felt lingering shame for the fact that his grand-relatives were Nazis, and admitted to Pomson that was part of the reason he didn't feel worthy to propose to her. Pomson assured him that she didn't care what his ancestors did. (the Disease)