You have to look at why Buchheim didn't like the film.
Taken from Wikipedia (so it must be true
)
Criticism by novel author Buchheim
Even though overwhelmed by the literally perfect technological accuracy of the film's set-design and port construction buildings, novel author Lothar-Günter Buchheim expressed great disappointment with Petersen's adaptation in a film review
[3] published in 1981, especially with Petersen's aesthetic vision for the film and the way the plot and the effects are, according to him, overdone and clichéd by the adaptation, as well as the hysterical over-acting of the cast he called highly unrealistic while acknowledging the cast's acting talent in general. Buchheim, after several attempts for an American adaptation had failed, had provided a script detailing his own narrative, cinematographical and photographical ideas as soon as Petersen was chosen as new director that would have amounted in full to a complete 6-hour epic, however Petersen turned him down because at the time the producers were aiming for a 90-minute feature for international release. Ironically, today's Director's Cut of
Das Boot amounts to over 200 minutes, and the complete TV version of the film to roughly 5 hours long.
Buchheim attacked specifically what he called Petersen's sacrificing of both realism and suspense in dialogue, narration and photography just for the sake of cheap dramatic thrills and action effects (for example, in reality one single exploding bolt of the boat's
pressure hull would have been enough for the whole crew to worry about the U-boat very likely being crushed by water pressure, while Petersen has several bolts loosening in various scenes).
Uttering deep concerns about the end result, Buchheim felt that unlike his clearly anti-war novel the adaptation was "another re-glorification and re-mystification"
[3] of German WWII U-boat war, German heroism and nationalism, and he called the film a cross between a "cheap, shallow American action flick"
[3] and a "contemporary German propaganda newsreel from World War II".
[3]
It appears Buchheim was reviewing the film cut, not the 5 hour TV series which was released later. Given that Buchheim himself wrote a 6 hour script and the 5 hour cut adds a lot of the suspense and boredom (for the crew) missing from the film, I think he would have been happier with the final result.
As for him saying the film glorified war, I can't agree with that at all. There's only a single character who ideologically approves of the war, and several, including the captain who speak openly against it. Nobody relishes the act of killing in the film. We see the crew enjoy themselves, but people who know they could die any day, tend to make the most of things. I've never fought in a war so I can't say what it's like, but this film didn't make we want to go off and be a submariner or think that it would be a cool thing to do.
I'd put it alongside Master and Commander in being realistic and authentic, but with a film plot because ultimately its entertainment not reality.