Maturity is the theme best suggested by Jerry's accomplishment in "Through the Tunnel"
Explanation:
Consider the case, when the story begins, Jerry is seen as a person who is not only close to his mother, but is also very dependent on her too.
At one point, he feels that he must become an independent person, due to which he strives for being accepted by boys who are much elder than him. For that he swam in the bay also tried to swim under water too claiming that if he does that he can also be like those elderly boys.