It was a movie, and despite whatever pro-British, anti-German bias and historical flaws it had, it was one of the best post-war movies in many other respects. It depicted the Admiralty War Room planning and chess game as well as could be expected, the personal sub-plots were perfect and not so overblown as to dominate the film, and the casting, acting direction, depiction of battle scenes were superb. If you want an anti-war war movie, this isn't it (try The Enemy Below from the same time frame or Gallipoli or Breaker Morant, from a later one). As for the history, well... you're rarely going to get unbiased truth in anything like a theatrical film and not even in many documentaries. If you cast this movie into the crapper because it smacks of propaganda, I guess you hated Casablanca as well.?
Note that in 1960, the War was still fresh in the minds of those who fought it. and unlike the USA, Britain was directly attacked, her cities bombed, her civilians killed. while I too protest the strongly anti-Nazi Kriegsmarine being portrayed as another fanatically loyal Nazi sect, recall that the British were fighting for their very lives at this point in the war: it was before the USSR and the USA got involved, and the U-boats were taking a massive toll on British supplies. it's also an issue of how Propaganda sticks in the historical memory of a population: even in the face of actual facts, the sheer volume of propaganda produced during wartime, as well as the popularity of major films, can still influence impressions of actual events.
Directed by: Lewis Gilbert
Writers: Edmund H. North
Starring: Kenneth More, Dana Wynter, Carl Möhner