In the following passage from Lord of The Flies, how does structure contribute to the atmosphere and theme? Ralph shuddered. The lagoon had protected them from the Pacific and for some reason only Jack had gone right down to the water on the other side. Now he saw the landsman's view of the swell and it seemed like the breathing of some stupendous creature. Slowly the waters sank among the rocks, revealing pink tables of granite, strange growths of coral, polyp, and weed. Down, down the waters went, whispering like the wind among the heads of the forest. There was one flat rock there, spread like a table, and the waters sucking down on the four weedy sides made them seem like cliffs. Then the sleeping leviathan breathed out—the waters rose, the weed streamed, and the water boiled over the table rock with a roar. There was no sense of the passage of waves, only this minute long fall and rise and fall.

This quotation is taken from the sixth chapter of William Golding's Lord of the Flies. Although the excerpt is narrated in the third person, we are looking at the scene through the eyes of Ralph. Because Ralph is among the most mature and civilized of the boys, he serves as a normative viewpoint, one whose perceptions are reliable and with whom the audience is likely to empathize.
At this point in the story, Samneric have been terrified by their sighting of "the beast," which is actually the corpse of a dead paratrooper. The boys are searching for the beast on the only part of the island they have not yet explored, a sort of peninsula connected to the rest of the island by a narrow natural bridge.     
At this point in the story, the boys are facing three obstacles. The first is nature itself, from which they need shelter. The second is the external "beast." The third is the "beast" within themselves. The water for Ralph seems to embody or foreshadow all three of these thematic antagonists. Structurally, the image serves a function of transition and foreshadowing; the "slumbering leviathan" suggests that the menaces that Ralph perceives have not yet fully awoken but, like the tide, are about to unleash their forces on the boys.
The table-like rock suggests something stable and civilized that will be drowned by the primal and powerful force of the ocean. The force of the tide and phrases such as "strange growths" also suggest something primeval and terrifying, contributing to the atmosphere of horror that is developing in the novel. Thematically, this also emphasizes that nature, both physical and human, is seen in the story as potentially immensely powerful and destructive when not tamed by civilization.