The oiler is the strongest and therefore the most likely one to survive the ordeal. Ironically, he is the only one who dies. This underscores the futility of his struggle against the indifferent forces of nature. That being said, this underscores the futility of all the characters' struggles. If the oiler is killed, and he was most likely to survive, then his death was the result of bad luck. Actually, "chance" is more appropriate here since we are dealing with the indifferent forces of nature. His death was random and in that singular context, his death lacks significance.
As the ordeal continues, the men seesaw between hope and hopelessness, and this is marked with bouts of frustration in dealing with a seemingly indifferent universe. The repeated phrase "If am going to be drowned" symbolizes this frustration. Note that the third person narrator ascribes this thought to all four characters.
It is also important to note that the oiler is the only character whose name is given: Billie. One would think this singles him out as a potential hero. But even singled out with a name, and even as the strongest character, he still dies.
Even the oiler's death is described objectively and without feeling.
In the shallows, face downward, lay the oiler. His forehead touched sand that was periodically, between each wave, clear of the sea.
The only sensible theme regarding humans and nature is that life and death are the logical ebb and flow of nature, represented by the oiler's head periodically being "clear of the sea." That is the significance of the oiler's death: a random consequence of nature's indifference.
The glimmer of hope is that, despite this futile struggle, the men do continue to struggle. In their camaraderie, all the characters share the futility of this struggle. This is a Realist and a Naturalist text. The forces of nature are not attributable to God or some cosmic force. Rather, the forces are simply the chaotic workings of nature. As the men struggle to survive, they are struggling with forces they know little about. So, even a theory like Darwinism will not give the men some logical sense of nature because in this case, even the "survival of the fittest" does not apply. Even the oiler, the fittest, is not safe from nature's indifference.