There are at least three themes in "Through the Tunnel":
In the beginning of the narrative, the eleven-year-old Jerry is attached to his mother, watching for the "yellow speck" on the beach under the orange umbrella, even when he swims with the native boys on the wild bay. But, when they disapprove of antics that have previously humored his mother, Jerry realizes he must assert his independence from his her. So, he swims to shore and demands swim goggles, nagging and insisting until he procures them. Later, when he desires the acceptance of the older boys and wants to feel mature himself, Jerry swims in the bay independently, and he practices going under water so that he can be like them.
Sensing that the older boys find him childish because they can do things he cannot, Jerry practices until he, too, can pass through the long underwater tunnel. When he finally attempts to pass through this tunnel, Jerry senses that his life is in danger because his lungs ache. Nevertheless, he perseveres,
He struggled on in the darkness between lapses into unconsciousness....His hands, groping forward, met nothing....
Finally, Jerry reaches the surface, "gasping like a fish." Successful, he knows he has matured.
He did not want them. He wanted nothing but to get back home and lie down.
Apparently without a father, Jerry spends most of his time with his mother. At least on this vacation, Jerry lacks friends, and his attempt to impress the native boys seems to indicate that Jerry is often alone. At the end of the story, more confident with himself, Jerry does not seem to mind being alone, however.