How does Jerry prepare for his task of swimming through the tunnel in "Through the Tunnel"?

Jerry practices for his rite of passage through the tunnel by conditioning himself to be able to hold his breath. He obtains swims goggles and practices going underwater and finding the tunnel so that he will know exactly where it is when he swims through it.
Jerry is an eleven-year-old English boy who goes on holiday with his mother on the coast of Africa. He is at that age between childhood and the teen years, an age at which a young person wants to be perceived as less a child and more as a young adult. Realizing this, his mother allows Jerry, who is a good swimmer, to go to the big beach, where there are great rocks. So, once he sees that his mother is on her beach, Jerry hurries the last few yards and rushes straight into the water where he starts swimming.After he watches the native boys diving down off the rocks, Jerry clowns in the water, hoping they will accept him. But, they ignore him until Jerry shouts a warning. Then, they "looked at him idly and turned their eyes back toward the water." After a while Jerry realizes that the boys are swimming through some gap or hole in a huge rock. He dives below the water to try to find the passage that they have used, but he cannot hold his breath long enough. Soon, he returns to the villa, and immediately when his mother arrives, Jerry demands swimming goggles. He pleads and nags until his mother takes him to a store and buys the goggles.Jerry now practices holding his breath, but when his nose bleeds at night, his mother orders him to go with her to the old beach. On the following day, Jerry rushes out early and returns to the new beach. The day's rest has helped him hold his breath longer, and he does better. He practices 

...as if everything, the whole of his life, all that he would become, depended upon it.

He dives down repeatedly until he finds the opening to the tunnel and he tries to find the opening in the rock until "he shot his feet out forward and they met no obstacle."
One day his mother announces that they will leave in four days, and Jerry decides that he will try to swim through the tunnel before he goes home. But when his nose bleeds badly, Jerry decides he will wait until he is older. Suddenly, though, he looks down into the water and decides "this was the moment when he would try."