A good place to start with this question would be to think about what Jerry himself has learned or gained from this experience - or perhaps more precisely, how he has changed as a result of going through the tunnel. This would give you a focus point to start your answer. I guess one of the things you would have to focus on is what Jerry is like before - think about his relationship with his mother and how he reacts to the boys he sees diving and the need he feels to be accepted - and then how he has after - again with his mother and his apparent lack of desire to be with the boys again. Of course, you will also need to spend time focussing on the actual journey through the tunnel itself - the sense of danger and the very real threat of drowning that Jerry underwent. And no doubt you will need to spend time closing with the sense of elation he felt on achieving his task! Hope this helps - just a few pointers to guide you.
If Jerry were with friends or his son years later, he might begin to describe his experience when he was eleven years old as his departure from boyhood. This description can be in the form of a monologue.
Here are some ideas to include in this monologue:
When he arrives with his mother on holiday at the beaches, Jerry tags along behind her. Still, he yearns to go to a wilder-looking beach where older boys dive and swim. Once there, he feels childish because he cannot dive through a long rock as they do. In addition, their looks when he acts silly to get attention make Jerry feel even more like a child.
After the older boys depart, Jerry tries holding his breath and finding the hole through which the others have swum. But panic fills him, and, defeated, he returns to the villa. Then, one morning his mother informs Jerry that they are soon departing for home.
Resolving to make this passage through the tunnel before he returns home, Jerry practices holding his breath until he has enough lung power to propel himself through the tunnel. When he gets goggles, Jerry tries traveling through the underwater passage, but he cannot make it far enough. However, after many hours of practice, aching lungs, feeling as though he is dying, and bleeding from his nose, Jerry goes back to the wild beach and finally succeeds.
Once recovered, Jerry sees the other boys diving and horsing around a half mile away. But now he no longer wants to be around them. He has made his rite of passage. Once home, Jerry does not even tell his mother. There is no need to tell her; he is content in his knowledge of himself. He has grown up.