The whole novel appears to be centered on the theme of perseverance. In their own ways both Santiago and his friend Manolin are subjects of perseverance. His very existence without a wife, the support of older friends and wealth, would have been a very good reason for Santiago to give up living. Even when his friend is taken away from him by his parents, he perseveres.
Manolin also perseveres in his friendship with Santiago despite the prevailing situations. As a poor man, Santiago barely has enough to eat. He does not throw in the towel but keeps his hope alive by fantasizing pots of yellow rice and fish. His fantasy is a source of perseverance for Santiago. Pushed by it, he is able to carry on with his trade virtually on an empty stomach. When the novel opens the old man has been at sea for over two months without any catch. The natural thing to do would have been to come back home, yet he stays at it. His hope is kept alive by the fact that previously; he had gone "eighty seven days without fish" after which he caught big ones daily for three weeks. When he hooks on to a massive fish which he has to pull in all by himself, he demonstrates his perseverance by hanging on even though badly bruised and battered.
The boy perseveres in his friendship with Santiago. He is detached from Santiago to join other fishermen who are well-to-do. This new situation would have been good reason to abandon Santiago, but he keeps on being his friend. The old man spends a considerable number of days at sea chasing the illusive catch. Meantime, the boy keeps a constant check on him through visits to his shack. The boy's perseverance pays off when Santiago finally does return. Unaided, Santiago has fought and killed sharks that have eaten his fish. The two friends finally reinforce each other despite the quality of the old man's catch. They show that perseverence can be rewarding.