Chapter 9 is the place to look for this particular sequence. The chapter begins with Simon discovering the dead man and parachute. After that, readers get an interesting and short sequence about Ralph and Piggy getting ready to head up to Jack's party. Ralph decides that they need to take a bath, so he jumps into the nearby water. Ralph playfully shoots some water at Piggy, and Piggy tells Ralph to knock it off. He doesn't want his glasses getting wet, because he doesn't want to have to clean them.
"Mind my specs," said Piggy. "If I get water on the glass I got to get out and clean 'em."
Ralph squirted again and missed. He laughed at Piggy, expecting him to retire meekly as usual and in pained silence. Instead, Piggy beat the water with his hands.
"Stop it!" he shouted. "D'you hear?"
Furiously he drove the water into Ralph's face.
Ralph, being a typical teenage boy, doesn't listen to Piggy. Piggy has never really stood up to anybody and defended himself, so Ralph thinks it's good fun to keep pestering him. Unexpectedly to readers and Ralph, Piggy lets it be known through his voice and his actions that he isn't going to stand being pestered and bullied anymore. He's demanding respect from Ralph, and the brief scene shows that Piggy's character is changing from somebody that the boys assume can be pushed around to somebody that is finally going to push back.