The answer to this question comes in the first section of this excellent tale of survival against the harsh elements of nature. As we are presented to the four men in their desperate situation, the narrator describes the scene and imagines how the men would have appeared as the day broke. He then imagines how strange the scene would have appeared if it were viewed from a balcony or from some distance. However, it is clear that they do not have the benefit of viewing their situation from a distance. They are so focused on their situation that they can only focus on the water and completely ignore what is happening above them. Note what the narrator tells us:
The sun swung steadily up the sky, and they knew it was broad day because the colour of teh sea changed from slate to emerald-green streaked with amber lights, and the foam was like tumbling snow. The process of the breaking day was unknown to them. They were aware only of this effect upon the colour of the waves that rolled toward them.
So intent are they on the sea that they do not even have time to look up and see what is happening in the sky above. They only know the time because of the effect of the sun on the colour of the sea.
The meaning of Stephen Crane's first line in his naturalistic story, "The Open Boat" is existentially explicated by the final sentence of his narrative:
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 the shore, and they felt that they could then be interpreters.
The men in the open boat, like all men, cannot understand an inscrutable and indifferent universe throughout their existential experience in the boat as they desperately try to reach shore and survive their dangers at sea. Crane writes that none of the men "knew" at the beginning; for, man is limited in his ability to understand nature. However, their experience with the forces of nature teaches them something about the nature of existence, and for this reason, they feel that they can be interpreters in, at least, a limited way after they reach shore. They do know, at least, that it is not a meaningful or righteous universe in which they exist.