In Doris Lessing's story, the mother feels extremely protective of her son Jerry since she does not have a husband. She gives Jerry an apologetic smile often and Jerry feels contrition. So, while she gives Jerry the freedom to swim at the new beach one day, she still worries:
She was determined to be neither possessive nor lacking in devotion. She went worrying off to her beach.
On her beach, Jerry's mother appears to Jerry as "a speck of yellow under an umbrella that looked like a slice of orange peel." While Jerry swims back to shore, he yet feels lonely--an indication that he wants to break from his mother, but feels his "contrition." Finally, however, Jerry conquers his fear in a daringly independent act. After completing this act, Jerry has more confidance and no longer feels contrition when he talks and interacts with his mother.
Thus, the rite of passage is Jerry's swimming through the tunnel underwater. His mother has afforded him the opportunity to do this, but, ultimately, Jerry has had to decide to grow up himself.