What do Ralph and Piggy find in the small lagoon in the book "Lord of the Flies"?

It's the conch.

“What’s that?”
Ralph had stopped smiling and was pointing into the lagoon. Something creamy lay among the ferny weeds.
“A stone.”
“No. A shell.’ ’ Suddenly Piggy was a-bubble with decorous excitement.
“S’right. It’s a shell! I seen one like that before. On someone’s back wall. A conch he called it. He used to blow it and then his mum would come. It’s ever so valuable—”

The key point is that it is Piggy who knows what it is called, and Piggy who knows how to use it. It's also Piggy who suggests using it to call the others together:

Ralph looked up.

“We can use this to call the others. Have a meeting. They’ll come when they hear us—” [said Piggy].

He beamed at Ralph.

Just like it appears as a symbol, though Ralph agrees, it is Piggy who really represents the conch: he is the one, right from the start, who makes it what it is. And when he dies, it shatters into a thousand pieces "and ceased to exist".

Hope it helps!