Santiago also battles against recent history and potential humiliation in The Old Man and the Sea. He has not caught a fish in nearly three months and the other villagers - some of them - laugh at Santiago and ridicule his failure and bad luck.
On the sea, he is struggling against his 80+ days of failure.
His greatest antagonist is himself. Santiago does not want to give up, but he knows he is facing danger and potential destruction. He worries that he will not make it.
There is little to add to such cogent answers as those above. The external antagonist is not just the sharks; it is Nature as Santiago battles the sea itself, the days and nights that pass before he reaches home. The internal antagonist that Santiago battles is Age, and his hand acts as a metaphor for this.
Santiago has to battle so many things in this story, and the sharks are only one of them. External conflicts include the weather, the marlin himself, and his own body. Internal conflicts are almost as significant. Santiago is alone and has to fight to stay focused over the course of this novella. Despite the discouragement of so many days without catching a fish, Santiago must fight against that and believe each day will be "the" day. The beauty of this story is that, despite all those internal and external conflicts, Santiago perseveres.
This is a very perceptive question. One of the remarkable aspects of this incredible novella is the way that conflict works on so many different levels. You are right in identifying the way that the sharks are definitely one antagonist that Santiago must face, however, let us also be aware of the many different opponents that face Santiago. The marlin itself of course is an opponent that he faces, as is the sea and the way that he has not been able to catch anything for so long. However, for me, the way I read the novel, the biggest conflict that Santiago faces is internal, as he struggles to believe in his ability to triumph and not to fail in the momentous conflict that he engages in against the marlin. Note how the memories of his former glory days help in this respect:
As the sun set he remembered, to give himself more confidence, the time in the tavern at Casablanca when he had played the hang game with the great negro from Cienfuegos who was the strongest man on the docks.
He intentionally is forced to remind himself of previous triumphs to give him the necessary confidence and determination now to succeed in perhaps the biggest struggle of his life. Having faced endless days of having caught nothing, and the derision and pity of other fisherman, he now has the chance to land a truly momentous catch of mythic proportions, but the biggest enemy he faces is within himself. Therefore, the sharks are definitely not the only protagonist that Santiago faces.
The answer to this question depends on how you interpret the conflict in this novella. On a literal level (man vs. beast), the sharks are living incarnations of Santiago's struggle, sadness, and failure. If you broaden your line of inquiry, however, one might argue that Santiago is likewise faced with an antagonist in the character of the giant marlin (again, man vs. beast). Looking through the same broad lens, you might argue that Santiago is -- at the heart of all things -- at war with himself. His personified conversations with his maimed hand (ala Tom Hanks and "Wilson" from "Castaway) serve as evidence that the protagonist has personified a non-human antagonist to serve as a foil (or sidekick, depending) in his broader struggle against himself.
Again, it all boils down to a question of perspective. And in any case, this is a great book.