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.