In one sense, William Golding's Lord of the Flies is anything but realistic. A plane crashes on a deserted island, everyone but the adults survive untouched, most of the children (except the choir) spend the night of the crash in different places (as evidenced by the fact that they all come streaming out from different places in the morning), and one of the boys (Jack) has a large knife with him. All of these details (and probably more) strain the bounds of credulity to some extent; however, readers (like movie-goers) are quickly ready to suspend their disbelief if the story is intriguing enough--and this one is.
In calling this a realistic novel, you undoubtedly mean the fact that these boys turn into savages when left unchecked by the restraints of laws or authority. We watch the transformation with horror and dismay, knowing that if the ship does not arrive when it does, all of the boys would have died. We also know that his is probably an accurate portrayal of the places our human nature takes us when we are not governed by any external forces.
The rescue is as realistic as anything else in the novel. The rescue serves several purposes, and one is the injection of hope. These boys, especially the older ones, will never be the same; yet they are alive and have a chance to become productive--rather than destructive--members of society.
Another purpose for the rescue in this very symbolic novel is to show that this war on the island ends, just as the actual war raging in the rest of the world ends. This events that happen on this island are a microcosm (small picture) of what is happening in the world around the island--World War II. The naval commander, though he is obviously quite unaware of what has actually happened on the island, says this:
“We saw your smoke. What have you been doing? Having a war or something?”
He intuitively recognizes what is happening here, though he does not know the extent of their "war."
Perhaps it is too neat and tidy in some respects to have the boys rescued "just in the nick of time," but if the story of these boys is an accurate reflection of what is happening in the "adult" world, there must be a rescue. If Golding had gone one step further and implied that the boys would all go on to live amazing, scar-free lives after their experience, he would have gone too far. Instead he is realistic and offers the possibility of hope for recovery and restoration after devastation and destruction.