Percival says the beast comes out of the sea. Why does this statement suddenly quiet the assembly of boys?

Percival's statement quiets the boys because the sea is such an unknown entity. It's huge and mysterious. On the island they can see what they're up against, but creatures from the sea are another story.

“He says the beast comes out of the sea.” The last laugh died away. Ralph turned involuntarily, a black, humped figure against the lagoon. The assembly looked with him, considered the vast stretches of water, the high sea beyond, unknown indigo of infinite possibility, heard silently the sough and whisper from the reef. Maurice spoke, so loudly that they jumped. “Daddy said they haven’t found all the animals in the sea yet.”