In Lessing's "Through the Tunnel," the bay is and represents something the boy isn't supposed to do or can't do; it is something the older boys can do, and the boy wants to do. The bay is basically a rite of passage. Or, more specifically, swimming through the tunnel in the bay is all of the above.
Notice that once he's done it, once he's accomplished his goal, he is in no hurry to go back. He's done it, he's made it, he's proved himself, he's matured. He's proven he belongs.
He begins the story as a little boy who seeks attention in any way possible, and desperately needs the approval of others. He matures and in the close of the story he is content in what he's done and in who he is.
The boy has to do it that day or he never will for two reasons. First, he is reaching the end of his physical limits. If he doesn't do it today he won't be in any shape to do it tomorrow. Second, he'll chicken out. He is causing himself pain and he is in pain, and if he doesn't swim the tunnel now he never will.
For Jerry, the tunnel symbolizes a major challenge. It symbolizes his passage from being a coddled young boy to being something more of a man.
The reason that he decides he must do it then or never is that he knows that he and his mom are going to have to go back home soon. If he does, without having swum the tunnel, he will have missed his chance to prove himself. We will not know if he will ever manage to really prove to himself that he is an independent young man and not a little boy.
The tunnel symbolizes the passage from boyhood to manhood. Jerry has tried to get through it and could not do it. He is aware that unless he accomplishes getting through it he will feel defeated. He knows that the other boys could do it.
Males tend to be competitive during adolescence especially. Jerry sees himself in a type of self competition. He has to prove to himself that he can and will get through the tunnel. Going through the tunnel is scary for Jerry and it is a fear he knows that he has to face now or never.