What does the wild bay represent for the boy?

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.