The usual interpretation is that the "safe beach" represents the comfort and security of Jerry's childhood and his mother and the rocky, "wild beach" represents the unknown adolescent future without his mother. The "tunnel" is a challenge or rite of passage or test Jerry must pass to gain access to that future.
His determination to be able to swim through the tunnel like the older French boys stems from a desire to test himself, to discover new abilities, both in his body and his will. It is telling that much of the story has to do with the analytical way Jerry goes about preparing himself for the attempt to swim through the tunnel; unlike the French boys, who seem to Jerry to effortlessly go through, he has to approach things very carefully. He approaches it like a job: he practices holding his breath, he thinks about how long he will need to be underwater, he even requisitions from his mother the necessary equipment (the goggles).
Of course his mother has no idea of what he is up to, but probably, in real terms, neither does Jerry despite his analysis of the task itself. He doesn't want to impress the French boys or anyone else. There is no sense from the story that he is interested in exploring the tunnel, on the contrary, he dreads meeting with an octopus in the tunnel. The process of preparing is uncomfortable and causes him to have tremendous nosebleeds. He just feels an unexamined need to do it. So he does.
Once he does, the spell is broken. The resolution of the story is that Jerry does not object when his mother tells him that he should not go back to the beach that day: "It was no longer of the least importance to go to the bay."
I think if we are to understand the tunnel as a "rite of passage," the nature of this rite says a great deal about Jerry and his relationship to his widowed mother. Jerry's motivation is completely internal. We can understand it as his desire to get away from his mother, to assert his independence, but actually nothing has been asserted, other than his own will. And the result of his adventure is to return to his mother. He has been changed, but not in the way you might think. He wants his mother to be proud of him: he blurts out at the end that he can hold his breath for two minutes, but his mother pays little attention. "Don't overdo it," she says. Jerry cannot tell her about the tunnel. He has a secret he keeps from her now.
A coming-of-age story, Doris Lessing's "Through the Tunnel" employs three major symbols to connote Jerry's state of being and his rite of passage. In the exposition of the narrative, as Jerry is with his mother at the "usual beach," which they frequent when they come on vacation, he looks over his shoulder at the wild bay, an action representing his growing desire to expand his horizons. Jerry's mother asks him, "Are you tired of the usual beach, Jerry?" Thus, it is apparent to Jerry's mother that her son wishes to "stretch his wings" and exert some independence by going to the wild-looking beach.
Then, when Jerry does swim out to the rocks and the wild bay, he leaves the protection of his mother, who became a "speck of yellow under an umbrella." When he sees the native boys, free in their nakedness, diving and swimming, Jerry envies them, and he craves their company. But, they are "big boys" and reject his childish antics in the water to get their attention:
"Look at me! Look!" and he began splashing and kicking in the water like a foolish dog.
After this rejection, Jerry desires to do what the older boys have done, that is, to swim under the water and pass through some type of gap, re-emerging far on the other side. This act, then, becomes Jerry's ultimate rite of passage to maturity, an act with which he becomes consumed. He tells his mother that he needs swimming goggles, then he practices to develop his lung capacity. Finally, he makes his attempt at passing through the tunnel and struggles, feeling that he is dying as he loses oxygen. But, at last, Jerry succeeds. "He did not want them. He wanted nothing but to get back home and lie down." Jerry has succeeded and proven that he, too, is a big boy.
"safe beach": the beach of Jerry's boyhood, safe with his mother nearby.
"wild beach": the unknown, the future that awaits the boy Jerry.
"tunnel": the passage from being a boy to being a "big boy," capable and gaining independence.
In Doris Lessing's "Through the Tunnel" the two beaches symbolize the two parts of Jerry that are in conflict. The safe beach, where his mother relaxes, represents the safety and maternal protection Jerry experiences as a child. The wild beach represents the part of Jerry that wishes to break forth from his mother's sheltering watch and become more independent. It is at the wild beach that he sees the boys swimming through an underwater tunnel. He decides to prove himself in the same way they do so he can feel a sense of belonging to a group of peers even though he may never actually join in with them. He sees what they can do, and he challenges himself through intense self-training to be like them. The tunnel represents Jerry's rite of passage as he moves from childhood toward independence.