Symbolism, Imagery, Allegory
We don't really have to do much work here, because Crane just comes out and tells us what the windmill represents:
This tower was a giant, standing with its back to the plight of the ants. It represented in a degree, to the correspondent, the serenity of nature amid the struggles of the individual—nature in the wind, and nature in the vision of men. She did not seem cruel to him, nor beneficent, nor treacherous, nor wise. But she was indifferent, flatly indifferent. (7.3)
That settled, we can turn to what it tells us about the correspondent. He seems to have gotten over his previous fury, when he was wanting to "throw bricks at the temple" (6.3). He accepts the face that the wind-mill has its back toward the men, they can't really do anything to get its attention.
He's even come to some sort of peace with this new universal order, talking here about "serenity." This represents another shift in way the correspondent views the world—nature is wholly apathetic to the suffering of man. This realization seems to help calm the correspondent's anxiety, as he begins to feel like the entire course of events, the sinking ship and the drifting lifeboat, was beyond anyone or anything's control. Nature didn't have it out for him after all. This unites the men and nature in a sort of brotherhood that had previously only existed between the men on the boat…and all thanks to some random windmill on the beach.