Who first introduces the notion that the beast comes from the sea?the book it comes from is lord of thhe flies

The name of the kid who first introduces this idea is Percival Wemys Madison.  He is one of the littleuns.  He does this in Chapter 5.

He introduces the idea while they are all having a meeting.  It seems that maybe they are going to stop believing in the beast, which would be a good thing.  People point out that they have been all over the island and had not seen the beast.

At that point, Percival whispers to Jack.  Jack tells the rest of them that Percival says the beast comes from the sea.