Piggy, Simon, and the airman being washed out to sea is a religious symbol of rebirth.
Burial at sea is often considered going back from whence we came. When a character crosses through water in a book, it is often symbolic of baptism or rebirth. In this case, the characters are freed and reborn by being washed out to sea.
The airman represents civilization and the adult world. It is the boys’ one connection to the word they came from, and it washes away. This demonstrates the change in the boys’ society from civilized to uncivilized.
When Simon is washed out to sea, it is because he is attacked by the gang pretending he is the pig. He is the beast to them. In reality, Simon is the prophet, the innocent being who is sacrificed in a Christ-like way. When Simon convenes with the Lord of the Flies “pig’s head on a stick,” he realizes that the beast is inside all of them.
“Fancy thinking the Beast was something you could hunt and kill! ...You knew, didn’t you? I’m part of you? Close, close, close! I’m the reason why it’s no go? Why things are what they are?” (ch 8)
When Simon goes to tell the others and warn them where the beast is, they kill him—mistaking him for the beast itself.
Piggy is sacrificed more intentionally. He represents civilization with his glasses and his brains. His thought processes are of the world they came from. When stealing his glasses for fire does not work, they roll a boulder on top of him and his boy is washed out to sea. At this point the men show up to rescue them, with the boys being reborn from his sacrifice.