what is the signifcance of both bodies ( simon and the pilot ) being washed away ?

I think the pilot was a strong sign that the boys need to quit being idiots. The dead pilot was not a beast but figurative message for Simon to relate to the boys. When they kill Simon they really kill any redemption that they might have had. Regardless of rescue, the boys are lost. Many people take this story as a Biblical allegory where God's saviour (Simon) and the tools Simon use (the Pilot) are taken away when the boys destroy their only hope.I don't think Simon was killed by mistake. The boys knew who he was while they stabbed him to death.