After the boys leave, Jerry sits on a rock and cries. Why is he crying?

(Through the Tunnel)

he envies their camaraderie and freedom and daring.

Explanation:

After his rejection by the older native boys, who have perceived him as immature as he splashes foolishly in the water, Jerry sits on the rough rock and cries "openly" because he envies their camaraderie and freedom and daring. So, after having "cried himself out," Jerry swims back to the beach where his mother sits, and demands swimming goggles, nagging her until she takes him to a shop and purchases them.