The main theme of "The Open Boat" is that man is all alone in a godless universe, where he is no less subject to chance and change than every other object in the cosmos.

Whatever man may think—and of course, this includes the men on the boat—he is not the center of the universe, not the most unique creature on the face of the earth. In the overall scheme of things, he's nothing, an insignificant speck in a vast, ever-expanding cosmos.

The men and some of the other characters in Crane's story can rant and rave to their hearts' content about fate and the workings of the universe, but that doesn't make the slightest bit of difference. Fate is as indifferent to them as it is to all individuals; it moves on its predetermined path without taking into consideration what men want.

Given this grim situation, it's no wonder that the Narrator succumbs to despair. Having the stark realization that neither God nor fate is about to come to the rescue, he can only feel alienated from the world around him, a world in which he once enjoyed a semblance of belonging. His further realization that the world isn't what he thought it was and that it is governed by complete indifference only makes his hopeless situation worse.