The basis of the conflict in the story The Old Man And The Sea is man versus circumstances. Santiago has a series of diverse circumstances which took away his opportunities.
First, Santiago had a conflict with the sea: The sea was his fountain of sustenance, and his job- Yet, it was not providing for it and, once it did give him the huge Marlin he so much wanted, it took it back from him, leaving him only with a skeleton.
Second, Santiago had a conflict with life: He was old, his energy was dwindling, and he was suffering the aches of age. Had he been a more fortunate and younger person he could have been able to extend his work day, or come up with other ways to obtain his fish. Yet, there he was, alone in the middle of the water.
However, the inspiration behind this man is our American society: Shaken but not beaten. This is a man to whom life had not been so kind, and yet, he is never beat down. In fact, he revels in the things he loves: Baseball, Joe DiMaggio, and the American dream (which is represented in the giant Marlin).
In Hemingway's The Old Man and the Sea, you've got the following conflicts:
Man vs. Nature: Santiago against the ocean and the sharks
Man vs. Himself: Santiago is a Christ-figure destined to suffer
Man vs. Man: Santiago's flashbacks to the arm wrestling matches foreshadow his struggles with the fish
Man vs. Society: Santiago is against the other fishermen in the village, who chide him because of his dry streak
Man vs. Family: Santiago vs. his daughter and the marlin, whom he calls brother
Land vs. Air vs. Sea: the three forces of nature are in conflict here as Santiago struggles to bring the marlin from sea to land
Authors vs. Book vs. Critics: just as Santiago struggles to bring in the marlin, so too does Hemingway struggle to complete his novella and have it live up to his and the critics' expectations. Santiago (Hemingway) fights the sharks (critics) over the marlin (the book).
The central struggle of the novella is the conflict between Santiago, the old man of the title, and the giant marlin that he catches after is has towed Santiago and his boat out to sea. Santiago struggles then for two days and two nights to bring the fish to land. Unfortunately, on the second day a large Mako shark bites a chunk out of the marlin which is strapped to the side of Santiago's boat. With blood in the water, more sharks feed on the marlin until there is almost nothing left and Santiago makes it back to shore with only the skeletal remains still attached to the side of his boat.
If you consider the conflict to be the external one, obviously the conflict is between the Old Man and the great fish that he hooks but cannot bring into the boat. The conflict drives the plot of the entire story and the action.
The internal conflicts are more numerous. The desire of the old man for respect from both the young man and the community, a desire to regain his luck, which has really run out, and the fight within himself when he has hooked the great fish and cannot decide whether to cut it loose and return home or to keep trying to bring it in despite the somewhat useless nature of the quest by the end of the story.