One may well ask about the significance of Jerry's age. As an eleven-year-old, Jerry is probably pre-pubescent and longing to no longer be considered a child. While he loves his mother, he watches her "naked white arm" whose hand once held his as they walked along the beach. Now, it no longer is connected to his physically, yet there is still a connection as she has not released him from his childhood psychologically.
Yet, she is "determined to be neither possessive nor lacking in devotion." So, she releases Jerry emotionally, and alllows him to swim out to the sharp rocks by himself. There, he encounters the Iranian boys who only accept him after he dives through the rocks as they have. But, when he cannot understand how they accomplish the feat of disappearing in the water and reappearing some distance, they frown their disapproval. So, Jerry practices after he asks his mother for goggles; he develops his lungs so that he can hold his breath long enough to pass through the tunnel that he finally discovers.
This passage through the tunnel is a rite of passage for the child of eleven to the burgeoning teen. When his mother asks him, "Have a nice morning?" with manly understatement, Jerry responds "Oh,yes, thank you." It is a "day of triumph" when Jerry passes through the tunnel. Now that he has accomplished his manly feat, "It was no longer of the least importance to go to the bay."