What do the tunnels symbolize for the young boy, and why does he decide that he will have to swim the tunnel?

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.