This movie shouldn't have worked. A group of victims, attacked by a Nazi sub in WWII, are surviving on a lifeboat, and the whole film takes place in the middle of the ocean, on that boat. It should have been dull, or at least had lulls. But Hitchcock wouldn't leave us high and dry like that.
Hitchcockian trick I learned in this film: Allow the audio focus on an intense dialogue, but point the camera at another event, hidden from those speaking. Sometimes I get so bored by a film that I'll wander off and let my eyes look at the internet while I listen to the dialogue or whatever. Can't do that with this master. He demands your full attention, every moment. And so much is happening on this lifeboat. Not just conflict and love... in fact the romance is pretty reserved. Hidden agendas, character revelations and inner struggles abound.
There is one thing that ruined this movie, and that's the main spoiler of the film: The Nazi is the bad guy. Oh, what a shock. A film in 1944 and the bad guy is the German? I rolled my eyes. Another German at the end is on the boat and they have to teach him morality, because, of course, Americans and British are more life-respecting than Nazis? Ethnic propaganda isn't worthy of Hitchcock. Sure, there's some hypocrisy there, because they brutally killed the first Nazi, but it could easily be seen as heated emotion, and so dismissed. I'm disappointed.
Apart from that, this film was perfect, everything I want from a Hitchcock, including a bit more than I expect.