Concerning Lessing's "Through the Tunnel," the story is told in third-person limited point of view. The thoughts of the mother and Jerry are revealed, but the thoughts of others, such as the older local boys, are not.
Another important convention central to the story is the use of setting, or in this case, two settings. A contrast is established between them. The safe beach is the mother's domain. It is a predictable, protected setting. The other beach is a wild bay, unprotected, unpredictable. It's not a safe tourist attraction. This is Jerry's domain.
The mother is trying to find a balance between protection and overprotection, concerning Jerry. Jerry is trying to establish independence and behave like older boys.
Jerry feels like an outsider and a little kid, compared to the local boys he sees at the wild bay. Swimming through the tunnel becomes a rite of passage for him. It proves he belongs and that he is a "big" boy. It becomes almost a quest for him. He wants to prove he can do it; he wants to tackle an obstacle. This is his motivation.