Have to agree with "bob the moo" on this one. I've seen it probably two dozen times, and will see it many more, I'm sure. One of my favorite "war" movies (w/Robert Mitchum and Curt Jurgens as destroyer and submarine captains, respectively). My other faves are "Platoon," "Full Metal Jacket," "Decision Before Dawn," (w/Oskar Werner and Richard Basehart--most excellent!), and of course, Kubrick's "Paths of Glory," w/Kirk Douglas, Adoph Menjou and George MacReady.
More than a blood-and-guts war movie (though it has plenty of terrific action scenes), it's more of a man-vs-man "cat and mouse" story. Absolutely excellent direction (by Dick Powell) and some terrific cinematography. Terrific scene between Jurgens and Theodore Bikel (who plays his Exec. Officer), that is nothing but facial gestures and eyebrows.
A very "moral" tale, it shows that even sworn enemies serving their countries in wartime can show respect and compassion toward each other, especially since they're all men-of-the-sea: sailors. Conversations between Mitchum and his ship's doctor and between Jurgens and his exec officer mirror each other. Note the scene of the American sailors pulling the German sailors aboard their lifeboats near the end; Mitchum helping rescue the U-boat captain and his exec after ramming them.
Great tale, well-scripted, acted, directed and shot. An model of how to make a terrific story into a movie.