I thought this one was a fairly decent movie until I read that Captain Prien never was a member of the German resistance - but in this movie, he is. The two most important parts of the plot are the war action and a completely fictitious story that shows Prien trying to free an old friend of his, a priest who has been arrested because he helped Jews. But Prien was in fact a member of the Nazi party and was never involved in the resistance. They should at least have added a disclaimer telling the viewers that this part of the movie is purely fictitious. Apart from this inaccuracy, I think the movie is enjoyable for people (like me) that like submarine movies. For example, the war action includes Prien's famous raid on Scapa Flow. But even the parts showing war action could have been better. The budget was obviously limited, so they included actual footage from WW II. This may sound interesting, but there are too many problems with that in my opinion. The footage looks much grainier than the rest of the movie, and when the footage changes from new to old, the editing is often not very good. For example, in one shot it's night, in the next it looks like dawn or dusk, then again like night. So the changes from new to old footage (and back) are always very / too noticeable.
On the plus side, there is some funny banter between the mariners. Among them: Harald Juhnke, an entertainer who is famous in Germany. He plays the cook. His scenes are funny, but I guess some of them will make those who like him a bit sad: in these scenes he drinks too much alcohol. Juhnke's alcoholism in real life was very probably the main cause of his death. He was born in 1929, and I think he would still be alive today (2008) if he had been able to control his alcoholism.
Also among the mariners: Ernst Reinhold as Lt. Raufuss (or Rauhfuß?), whom fans of old submarine movies might remember from 'Haie und kleine Fische' (1957). If you haven't seen that one: I liked it better than 'U 47', so I think I can recommend it if you like this kind of movie. I don't think it's a great movie, but it's also about submarines, there's more (and funnier) banter, and it isn't as inaccurate as 'U 47'. (Or, to be more precise, it is a fictitious story that doesn't use the names of real historic persons when such names should not be used.)
