The story "Through the Tunnel" is a rite of passage story. The protagonist, Jerry, is growing up. He wants to establish more independence from his mother. She isn't a bad mother; Jerry simply wants a bit more freedom. His first steps toward that independence are symbolized by the two beaches. There is the safe beach that his mother is on. It is also the beach that she would like him to stay on. The other beach is the wild beach with the native boys. Jerry sees being there as a step toward becoming his own man.
While at the wild beach, Jerry sees a group of boys diving down into the water and swimming through a long dark tunnel.
He could see the hole. It was an irregular, dark gap; but he could not see deep into it.
Jerry might be at the wild beach, but it is clear that he is not a part of the group of boys, because he can't make it through the tunnel. Making it through the tunnel becomes Jerry's singular goal for the rest of the story. It is symbolic of his transition from boyhood to manhood. Jerry can be at the wild beach, but as long as he doesn't swim through the tunnel, he isn't fully his own man. That dark underwater hole in the rock is the "doorway" that Jerry must step through in order to gain entrance into independent adulthood.
He knew he must find his way through that cave, or hole, or tunnel, and out the other side.