Modernism: Although it is a far cry from the "lost generation" of Hemingway's earlier novels, The Old Man and the Sea does show how a man is alienated from society, the problems of existential identity and inevitable suffering, and the intense subjectivity of a character who reveals himself through interior monologue.
Santiago indeed is like an existential Christ-figure, cut off from family, other fishermen, his disciple Manolin, and his ability to redeem his former greatness. The sharks seem to be the inevitable critics of society's cruel ambivalence toward the artist.
Realism: the novel is simply told a plain style: simple diction, simple sentences, few adjectives, no hyperbole. Its protagonist is a humble fisherman who has many weaknesses. The novel seems to distrust passion, irrationality, and emotionalism in its style and characterization. It is a simple, straight-forward allegory and morality tale, devoid of authorial intrusion or grandiose depictions of nature.