What do you think is the main message, or theme, of this story? Consider what the swim through the tunnel means to Jerry and why Jerry no longer feels he has to go to the bay.
Concerning Lessing's "Through the Tunnel," you should be cautious if you're looking for a cliche or a one-liner that tells the reader how to live life. Fables and other kinds of literature do that, but sophisticated contemporary literature usually does not. Life is more ambiguous than that, and sophisticated literature usually reflects that.
Theme in "Through the Tunnel" relates to the ideas raised.
First, the issues of parental control and protection and a child's independence are raised. Jerry is apparently a little overprotected at the beginning of the story and his mom knows this. The mother struggles with trying to give Jerry some freedom, and Jerry struggles with trying to be independent.
Second, Jerry's self-determination is featured. He is immature at the beginning of the story, but grows tremendously during the course of the plot. He trains himself in order to swim through the tunnel, and succeeds in doing so. He accomplishes his goal in spite of great physical obstacles. He has grown up.
In the resolution, Jerry is no longer interested in going to the bay because he no longer needs to. He accomplished what he needed to, and proved what he needed to.
The main theme of this story is growing up, coming of age, becoming competent. In the story, Jerry goes from being something of a wimp who has no confidence to being a guy who is quite proud of himself.
The swim through the tunnel means a lot to Jerry. It means that he has accomplished somethng very difficult which took a great deal of effort and courage.
Because he feels so good about himself, he no longer has anything to prove and does not need to go to the bay. He no longer needs to go there as a way to prove his independence from his mother.