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 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. 

Thus begins Stephen Crane's short story The Open Boat. The author paints a picture of a menacing slate gray ocean where razor sharp waves rise and fall --- imagery.