Do any of the boys believe that the "beast" is real? When they find out, who was it really? What happened to the pilot? Did he survive? Die? Explain

Yes. In fact, it turned out to be the pilot. He died.

The boys slowly believe in the beast. Their understanding of the beast is the dead pilot in the tree. Simon's idea is really themselves "at once heroic and sick". The dead pilot eventually floats back out to sea.