"NONE of them knew the color of the sky. Their eyes glanced level, and were fastened upon the waves that swept toward them. These waves were of the hue of slate, save for the tops, which were of foaming white, and all of the men knew the colors of the sea. The horizon narrowed and widened, and dipped and rose, and at all times its edge was jagged with waves that seemed thrust up in points like rocks. " (Stephen Crane; "The Open Boat, pg 728)
In the first sentence of the story we are drawn into the struggle of the survivors of a boat in a storm so bad that they can't do anything but row and bail. They can't even see the sky, all they can see is wave after wave. It is night for one thing. The very next part of the story indicates that the sun begins to rise. This opening sentence makes the reader realize that the situation is intense and we, like the occupants of the boat are in for a terrible ordeal. They are intent and focused so hard on the waves that they share with the reader their desire to live.
I believe that it gives us the knowledge that everything is dark and the sky and water are blending together. They could not tell the sky from the water. They were in the midst of both.
They were so occupied with staying alive and avoiding being swamped that they didn't have time to look up.