Here's a bit from the EbscoHost database. The quote is from

Villains, victims, and veterans: Buchheim's `Das Boot' and the problem of the hybrid novel-memoir... By: Thompson, David G.. Twentieth Century Literature, Spring93, Vol. 39 Issue 1, p59, 20p

To the extent that one can check Das Boot against independently established historical "facts," it is essentially accurate. Probably its only considerable distortion results from the attempt to represent nearly the entire U-boat campaign in a single patrol, which tends to distract attention from the author's outstanding point: the futility of the last phase, 1943-1945. For all that, the story's most important elements may be the things which cannot be proved or disproved: the personal impressions of an individual observer. U-boat veterans surely will continue to disagree about this; but perhaps the final word should belong to Kapitanleutnant Heinrich Lehmann-Willenbrock, the actual commander of U-96, upon whom Buchheim based his character the Old Man. Unlike his fictional counterpart (and so many real German submariners), Lehmann-Willenbrock survived the war and read Das Boot in 1973. Writing to his old comrade "Lothar" that summer, he concluded that the book had been

"...well worth the effort. Today's reader will find out and see for himself what kind of men the submariners were, and what their fortitude and "heroism" were really about. Certainly there will be a lot of criticism from our ranks, because it is unpleasant to take that gently-dusted, gold-framed picture under the magnifying glass again. And the Donitz myth is spoken with such reverence. It's just a question of whether one should concern himself over it, or simply let that generation pass into eternity without being understood."

"P.S.," he added with apparent humor: "It's a good thing you made clear in advance that the characters portrayed did not really exist" (Qtd. in Salewski 132).
Salewski, Michael. Von der Wirklichkeit des Krieges: Analysen und Kontroversen zu Buchheims >Boot<. Munich: Deutscher Taschenbuch, 1976.