Well, I am building a type VIIc U-boat,but I don't think that it will get bigger than 3 feet (90cm?), but the first time I saw the movie, I thought the surface scenes were filmed with a model too, as for the underwater scenes I thought it was more of a computerized 3D animation in that kind of thing.
I can't really say that the movie was bad, since I'm not really a u-boat expert, just enjoyin' the show! The music was really good in my opinion, especially when the boat cruises the water. Not bad!