As well as his use of metaphor and simile, Golding's use of epithets for the boys is significant. Immediately Ralph, the golden-haired leader, delights in a name that the fat boy abhors: Piggy. Thenceforth, the boy who looks adultlike with his myopia and thinning hair is known by this epithet. This name is extremely symbolic too, as Piggy, like the sow, in Chapter 11 is slaughtered. His death signifies the total rejection of adult-like society, reason, and order. Another boy, the one with the mulberry mark who is immediately lost is not even named, and the indistinguishable twins, are given an elided epithet, Samneric denoting the insignifiance of their individuality. There are simply manipulated by the older boys, specifically Jack who terrifies them (they "protested out of the heart of civilization"--metaphor) and forces them to join with the hunters. Then, as Jack degenerates into savagery, Golding uses such epithets for him as "savage" and "the green and black mask" (mask is also a metaphor for the savage nature covering the civilized one). Even Ralph calls Jack "a beast and a swine and a bloody, bloody thief" and the others "painted fools." (Ch 11)
An interesting metaphor is the description of the water as a "leviathan," (ch 6) hitting the rocks sending a "thunderous plume of spray leaping half-way up the cliff." The rocks and spray as metaphor for a whale come to mind later when Piggy is killed and his head dashes against this rock and water--so powerful--that takes Piggy's life.
Of course, the final simile, uttered in dramatic irony by the rescuing officer, is his surmising that the boys have experienced an adventure "Like the Coral Island," a children's adventure story.
There's lots of ways to answer this, and of course, Golding uses different techniques at different times to establish different tones. His use, particularly of natural imagery and metaphors, to inform the tone of the novel becomes more and more sinister as the novel descends from civilisation to savagery.
As an example, here's the first paragraph:
The boy with fair hair lowered himself down the last few feet of rock and began to pick his way towards the lagoon... All round him the long scar smashed into the jungle was a bath of heat. He was clambering heavily among the creepers and broken trunks when a bird, a vision of red and yellow, flashed upwards with a witch-like cry; and this cry was echoed by another.
An unusual metaphor in the middle of this paragraph gives us - even very early on in the novel - a sense of one of the key themes. The mark the crashing plane has left in the forest is "a long scar", a badge, perhaps of the way human evil is considered "natural" in this novel: the "scar" cut into the paradise an emblem of the "darkness of man's heart". It's also a personification of the island, giving it a human "scar" - as if, like the Beast is supposed to, it has some sort of anthropomorphic properties. You also see the ominous nature of the natural island in the frightening simile "witch-like cry" of the birds.