The oiler (also known as Billie) dying at the end of the story is an example of situational irony. His character is physically strong, so when all the men have to swim to the shore lest they drown, he is far ahead of the others. The reader would naturally expect for the most physically fit and most hardworking character to survive. He is even the only one to have a name, which is a symbol of humanity. The name may be taken as a sign of importance, a mark that suggests the named character is less likely to die and more likely to succeed than the unnamed characters.
Crane subverts the reader's expectation, however, by having Billie be the only one of the men to die. This choice heightens the tragic aspect of the story and emphasizes many of the main themes.
As much as readers might have sympathized with the oiler up to this point, Crane's point is that the world does not care who deserves to die. Fate is arbitrary and mortality is the only certainty. No matter how strong or good a person is, they are still vulnerable to death, because they are mortal. No matter how weak or bad one is, they are still able to be saved by fate, because fate is random. Crane uses situational irony to play off the reader’s expectations in order to rebuff and subvert them.