A symbol is something "real" or concrete that stands for an idea, so the story itself is not a symbol. However, within the story is a major symbol that communicates the story's theme: the tunnel. The tunnel is a dark and dangerous place; to swim through it requires great skill and courage. Within the context of the story, the tunnel can be interpreted as symbolizing the passage from childhood to adulthood.
At the beginning of the story, Jerry is a child whose mother treats him as a child, trying to protect him from the world. When Jerry meets the older boys on the "wild" beach, he feels humiliated by his childishness and weakness in contrast to their strength and confidence as they routinely swim through the water-filled tunnel off shore. Deciding to make the trip through the tunnel himself, Jerry disciplines himself and trains daily for the challenge, which turns out to be very dangerous. Jerry does succeed, however, and feels a much greater confidence in himself. He has gained self-respect and does not need to tell others--the other boys or his mother--what he has achieved.
Jerry's swimming through the tunnel can be interpreted, then, as making the passage into adulthood, something everyone experiences. The story develops the theme that becoming an adult requires courage, determination, and self-discipline, but the reward is a feeling of accomplishment, self-confidence, and self-respect.