Three is a magic number in the novella: three days, three levels of nature, and three parts. The novella can be read as tragedy, so a reader may apply Aristotle's cause-effect triangle. I teach the novella using rhetorical analysis: the novel is the fish and Hemingway is the old man and the sharks are his critics. This conforms to Aristotle's other triangle: ethos (writer), pathos (reader), logos (text). In other words, the writer can never please his ideal reader, or himself, with the book, only bring back a great big skeleton of one.
Day one = Inciting Incident = Preparing to write. Hemingway focuses on land, as Santiago prepares to go to "way out" to sea. The boy, his disciple, contrasts with the old man and the other fisherman. The boy will reappear in the end.
Day two = Peripeteia (turning point) and Anagnorisis (climax) = Writing the Book. Focus here is on the sea as it contrasts the land and air. The fish is analogous to Santiago (brothers), and it contrasts also with the birds above and the sharks below. The turning point and climax come in killing the fish and the remorse thereafter.
Day three = Catastrophe and Resolution = Defending the book after publishing. The focus here is on fighting the sharks. It ends with the resolution that the old man, a Christ-figure, becoming aware of his suffering and potential, symbolized by the skeleton.