It hasn't always got good reviews. On Rotten Tomatoes one critic called it dated propaganda, while the New York Times wrote it down as a documentary rather than drama. I think both miss the point, though it could be a question of taste, as I found The Cruel Sea both moving and gripping.
Too often mainstream movies value scenes with bangs rather than creating a film of depth. Take U-571, which has implausible battle scenes and a much protested re-writing of history, yet critical consensus by Rotten Tomatoes as "a tense thriller". But in practice it is so false as to be truly describable as propaganda (and I suspect it will date badly), whereas The Cruel Sea feels as if it captures the reality of the Battle of the Atlantic.
The Cruel Sea works through gradualism: whereas each scene might have lower impact it builds to a greater impression. It's opening line is a great one:
This is a story of the Battle of the Atlantic, the story of the ocean, two ships, and a handful of men. The men are the heroes; the heroines the ships. The only villain is the sea, the cruel sea that man has made more cruel
It captures the energy draining relentlessness of years of war, hard days and black nights at sea, the reality of war as a marathon not a sprint.