Naturalism is a genre in which nature and humans are, at times, adversarial to each other. The other times, nature is presented as completely uncaring of the existence of humans. Naturalism can often be a cold and realistic way of looking at nature. Crane lets readers feel this right from the very beginning of this story.

These waves were most wrongfully and barbarously abrupt and tall, and each froth-top was a problem in small-boat navigation.

This quote paints a picture that the waves and the boat are in a battle against each other. The waves are not something that the boat handles in stride. The waves are a problem for the boat, and the boat is struggling to hold its own.

Another good quote is the following:

A singular disadvantage of the sea lies in the fact that after successfully surmounting one wave you discover that there is another behind it just as important and just as nervously anxious to do something effective in the way of swamping boats.

This quote does a good job of showing just how relentless and uncaring nature can be. The men and the boat might successfully win the battle with a particular wave, but nature is eventually going to win out because of its relentlessness. There will be wave after wave after wave.

There is a great quote in section six that very obviously points out to readers that nature is utterly uncaring toward the struggles of humans.

When it occurs to a man that nature does not regard him as important, and that she feels she would not maim the universe by disposing of him, he at first wishes to throw bricks at the temple, and he hates deeply the fact that there are no bricks and no temples.

The final line in the first paragraph help to convey the story's Naturalist theme: "These waves were most wrongfully and barbarously abrupt and tall, and each froth-top was a problem in small-boat navigation." Here, we understand just how nature can be seen to be wrongful and barbarous to humanity because humanity is of no concern to Nature. Nature does what it does, and what we want or hope for is futile unless this was what was already going to happen. The waves are relentless, coming quickly and pushing the boat up and down, up and down, causing such problems for the four passengers. Their comfort, even their lives, are of no consequence to the ocean.

Still, another quotation seems to personify the sea, an element of nature, even more clearly as an antagonist: "A singular disadvantage of the sea lies in the fact that after successfully surmounting one wave you discover that there is another behind it just as important and just as nervously anxious to do something effective in the way of swamping boats." Here, the sea seems to be anticipating its actions, purposefully trying to overwhelm the small boat in which the men are adrift. How can four small men possibly hold their own against the whole sea? They cannot.

The central theme of Naturalist style is that man is at the mercy of his environment.  This theme is evident in several quotes depicting the men in the vulnerable open boat being buffeted by the sea, the inexhorable force of nature.

In the opening paragraphs of the story, the injured captain thinks about the ineffectiveness of the actions of "even the bravest and most enduring" in general terms when he recalls man's utter helplessness when, 

"willy-nilly, the firm fails, the army loses, the ship goes down" (Section 1). 

Free will and human efforts count for little in the face of the harsh realities of the natural world.

In the following section, the author creates a vivid image of the little boat with four helpless men bobbing in the water, at the mercy of the sea around her:

"She seemed just a wee thing wallowing, miraculously top up, at the mercy of five oceans.  Occasionally a great spread of water, like white flames, swarmed into her",

while the tiny, insignificant men, cognizant of their position in the scheme of things, "bail...serenely" (Section 2).

As their little boat is tossed by the furious waves, the men cling to any hope they can find, but in the end they know they will survive only if the elements allow them to.  Their conversation reflects this feeling, when they say,

"we'll get ashore all right...if this wind holds...if we don't catch hell in the surf" (Section 2).