The premises which have been stated are certainly a launching point for the essay. With its Modernist aesthetic of tension, ambiguity, irony, allusion, and an end to Santiago's prowess as a fisherman, The Old Man and the Sea by Ernest Hemingway clearly defines a work of this genre. And, the Modernist aesthetic overrides the realistic elements of close attention to detail, objectivity, and simplicity as these elements are enclosed in the dream of the lions and the sense of drifting for days that both take away any certainty or realistic detail.
Perhaps, with the overriding metaphor of the dream of the childhood experience of the lions and the ways that Santiago's mind creates his world as the focal point of your essay, you could demonstrate how the Modernist elements and realistic detail develop Santiago's isolation, disillusionment, and lack of meaning in his life. At any rate, you may want some unified motif or theme that will connect your discussion of the various elements.
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.