Jerry is a boy at the threshold of manhood who is lovingly attached to his mother, yet he feels the pulling desire of more manly pursuits.
As is often the case, because his mother is a widow, now alone with her son, Jerry feels the need to be chivalrous toward her, but he also wishes to be independent. When she gives her permission for him to go to another beach, he feels a little guilty:
And he almost ran after her again, feeling it unbearable that she should go by herself, but he did not.
After he swims out to the rocks, Jerry still has conflicting feelings and looks back for her:
When he was so far out that he could look back...past the promontory...he floated on the buoyant surface and looked for his mother. There she was, a speck of yellow under an umbrella...
It is not until he feels humiliated by the "dark boys" that scoff at him and swim away that Jerry is prompted to learn to pass through the tunnel as a rite of passage. When he finally does this, "[V]ictory filled him," and he feels the equal of the other boys and "[H]e did not want them." Instead, Jerry returns to his mother, confident in his new manhood in which he can still be his mother's son, but know he can achieve adult challenges. For, when his mother tells him that he should not swim any more that day, she prepares for "a battle of wills," but Jerry does not argue since "[I]t was no longer of the least importance to go to the bay."
This failure to respond to her suggestion,"I wouldn't overdo it, dear," points to the independence that Jerry has achieved as a result of his achievement. Jerry has become more adult because he now has secrets from his mother and is able to engage in dangerous activities, away from his mother's protection. He is no longer dependent upon her and has completed his rite of passage into manhood.