It is important to understand that this text is an example of Naturalism, which is a philosophical belief in the insignificance and powerlessness of man compared to the might and power of nature. This text reminds us of the way in which, in spite of all of our achievements, nature is still so much more mighty than us. What is interesting about the way in which the oiler dies is that actually he had the strongest arms out of all of the crew of the boat, so we would have thought that he would have made it to the shore safely. However, his death and the arrival of the correspondent instead indicates the way in which it is a matter of chance and nothing more that guarantees our safety: man is puny when faced with the sheer majesty of nature, and our strength cannot compare. Note the way that the two kind of receptions the men received is indicated in the following quote:
It seems that instantly the beach was populated with men with blankets, clothes, and flasks, and women with coffee-pots and all the remedies sacred to their minds. The welcome of the land to the men from the sea was war and generous; but a still and dripping shape was carried slowly up the beach, and the land's welcome for it could only be the different and sinister hospitality of the grave.
The oiler dies therefore to emphasise the random nature of the survival. The fact that he, the strongest of the four passengers, was the one to die, underlines the fact that it was sheer chance that let the others survive. Nature is a force that still remains more powerful than us.
"The Open Boat" is actually an adventure story. It was published in his first collection of short stories which, significantly, was titled The Open Boat and Other Tales of Adventure (1898) People read "The Open Boat" for enjoyment, not for the philosophical implications. They share vicariously in the experiences of the men who are trying to save themselves from death at sea. But is is a more serious adventure story than the typical adventure stories in which there are usually happy endings. Stephen Crane probably decided to have one of the four occupants of the lifeboat die in the surf mainly to show that there had a very real and present danger all along. They were in danger of capsizing out at sea and then in danger of being drowned in the surf when they almost reach land. Two of the men are saved by a man who takes his clothes off and charges out into the surf to help them. Evidently the shipwrecked men are too weak to cope with this last challenge posed by the heavy surf.
Then he saw the man who had been running and undressing, and undressing and running, come bounding into the water. He dragged ashore the cook, and then waded toward the captain, but the captain waved him away, and sent him to the correspondent. He was naked, naked as a tree in winter, but a halo was about his head, and he shone like a saint. He gave a strong pull, and a long drag, and a bully heave at the correspondent's hand.
The rescuer saves the cook and the correspondent because they happen to be the closest to him. The captain declines being rescued because, as captain, he feels responsible for the others. However, it should be noted that when the rescuer wades toward the captain in the heavy surf he is using up valuable time. The oiler is not saved simply because he happens to be the farthest away and drowned before the rescuer could get to him. There is no peculiar significance to the fact that it was the oiler who drowned. If nobody had drowned, then it would have detracted from the seriousness of the danger as well as from the seriousness of the story. As long as the correspondent is rescued, the ending is satisfactory to the reader, because the reader has been identifying with the correspondent all along. The reader has been sharing the correspondent's fears and thoughts about the meaninglessness of life. The correspondent needs to be alive at the end because the story is told from his point of view.