While at first it may appear that the type of conflict in this story is strictly man vs. nature, the true conflict is much deeper. Santiago, a lifelong fisherman, is growing old, and feels the need to prove himself by landing a gigantic Marlin-type fish. All the while, Santiago contends with the rigors of his old age, and keeps facing the need to fortify himself internally. In essence, the story's main conflict is indeed an internal one -- Man vs. self. All this is seen through the eyes of the boy, who idolizes the old man despite his increasing infirmities. In the end, the gigantic fish is caught, but must be strapped to the side of the boat, and as a result, Santiago returns to shore with nothing more than a frayed, worn, and eaten skeleton of the glorious prize fish. The outcome is ironic and sad all at the same time.
In the book "The Old Man and the Sea," Santiago had been a good fisherman all his life. The boy idolizes him and knows that he had great skills. He wants to learn these skills from Santiago. However, Santiago has not been able to catch any fish for a good length of time. To himself and the other fishermen he is starting to look like a failure.
Santiago goes out into the deeper sea alone. He catches a majestic fish, but it is too big to get into the boat. He straps the fish to the side of the boat. The journey home is fraught with peril for Santiago and the fish. He fights off the sharks and other predators for the fish. He can not give up. He must bring the fish back so he can prove to himself that he is still a man.
While the main conflict may appear that to be the old man fighting the sea creatures and elements to bring back the fish, the real conflict is an inner conflict between the old man and himself. He has to prove to himself that he is still a good fisherman.
In the end of the story, Santiago makes it back with not much more than a carcass and skeleton of a fish.