Hemingway's short novel is a great example of a clean, literary narrative. The story is presents multiple layers of conflict, narrative and thematic. 

Santiago is engaged in a struggle for dignity while he is out at sea, fighting to capture the largest fish of his life and to regain some stature in his community. The fish is necessarily read as symbolic, standing for the dignity and worldly success that have eluded Santiago in recent years.

Phillip Young put it this way: 

"It is the knowledge that a simple man is capable of such decency, dignity and even heroism, and that his struggle can be seen in heroic terms, that largely distinguishes this book."

Though he has been challenged to gain material and social success, Santiago has gained knowledge, expertise and friendship. He is a spiritual success, so to speak, and catching the marlin after days of struggle stands as proof of his success in these areas. 

Returning with the skeleton of a great fish, Santiago returns to his village, again, as a spiritual success and as a material failure. The text is open to this rich, multi-layered reading, wherein meaning exists on at least two levels, symbolic and literal. 

Hemingway responded this way when asked about symbolism in the novel:

He once said that he “tried to make a real old man, a real boy, a real sea and a real fish and real sharks. But if I made them good and true enough they would mean many things.”

Formally, the novel is a masterly example of controlled narrative perspective. Though the novel is written in a third person narrative perspective, much of the story is related in first person "internal dialogue" where Santiago speaks silently to himself. He also speaks to himself, to the fish and to a bird in his boat.

This use of dialogue helps to both characterize Santiago (with indirect characterization) and keeps the narrative dynamic. 

A taut novel demonstrating the strength of human will, this Hemingway work stands as an example of effective narrative form drawing a compelling story out of the material of a single day and a single life.