The big question about the ending surrounds the death of Billie the oiler. Why does he die? In his final moments, he tries to defeat nature by strength while the others use logic and reason. He also sort of abandons the others, too, leaving them floundering as he makes his way to shore. Maybe his death is some sort of Reverse-Darwinism (as in, Survival of the Not-Fittest, or Non-Survival of the Fittest).
Crane is very careful not to suggest anything one way or the other. He leaves it all up for interpretation. He barely even tells us that the oiler is dead—all we learn is that he's found "face downward" in the shallows, and then, two paragraphs later, he is a "still and dripping thing" that is carried to shore (7.36, 38). For all we know, the oiler might have just taken a moment to do some snorkeling, and the others found a dead fish floating off shore. Unlikely? Yes, but not impossible.
All jokes aside, the oiler does die, and we are left to wonder the reason—if there is even a reason at all. On the flip side, we can ask why the other three survive, but with similarly unsatisfactory conclusions.
Let's focus on what we do know: a man spots them falling out of the boat and comes running down the beach to rescue them. He strips off his clothes, as if returning to a state of bare purity. As soon as he is naked, the correspondent notes "a halo" over his head, and sees him "[shining] like a saint" (7.35). This description seems to be hinting at the existence of a sort of inherent goodness to humanity, perhaps intended as a contrast to the cruelty/indifference of nature. To really drive this point home, a crowd of other rescuers descends on the scene, bearing all manner of warm, nourishing and life-sustaining gifts.
The final mystery comes at the very end. The men are lying awake on shore (we tend to imagine them hanging in hammocks and drinking out of coconuts) and they hear the sound of the ocean. We'll let Crane take it from here:
When it came night, the white waves paced to and fro in the moonlight, and the wind brought the sound of the great sea's voice to the men on shore, and they felt that they could then be interpreters. (7.39)
We are left to wonder: why can they be interpreters now? We like to think it has something to do with their realization that nature doesn't have it in for them, that it acts with indifference to human beings (this was in the days before climate change, remember). But what exactly is it that the sea is hoping to communicate to the rest of humankind? Any ideas?