While the Nobel Prize in Literature is for a writer's life work, there are nine Literature Laureates for whom the Swedish Academy singled out a specific work for particular recognition.
The Nobel Prize in Literature 1954 was awarded to Ernest Hemingway "for his mastery of the art of narrative, most recently demonstrated in The Old Man and the Sea, and for the influence that he has exerted on contemporary style".
I read this as a corroboration of the generally understood principle that this Prize was awarded to Hemingway for more than just one book. Citing his influence on contemporary style, it seems the committee was awarding Hemingway the prize for his career achievements as much as for this book, which also won the Pulitzer Prize.
It could be that this book, fresh in the public mind, was the catalyst for the committee's decision to give Hemingway the award.
Though The Old Man and the Sea is not Hemingway's best book, it does demonstrate his strengths with clearly drawn drama, complex emotion, and clean prose style. (There is at least one unbelievable bit toward the beginning when Manolin describes his first fishing experiences. It's too stylized, too odd, and too forced to get a pass. The rest of the novel though is quite readable, compelling, interesting, and nicely open to interpretation.)