Hemingway has used a number of literary devices in the entire novel for creating a pictorial world and give an account of Santiago’s world and his great struggle.
Some critics call this story as an allegory by comparing suffering of Santiago to Jesus’ sufferings. Author features alliteration while describing “flying fish” and “full-blooded fish” that Santiago catches. You can see both alliteration and onomatopoeia in Hemingway’s description of fish,
“left the water and the hissing that their stiff set wings made as they soared away.”
I have underlined the words using alliteration and onomatopoeia in italics.