Doris Lessing offers Jerry's experiences as a coming-of-age story in which the boy develops personal skills and gains a sense of pride and achievement. Jerry is inspired to embark on the mission to swim through the tunnel by his interactions with the local boys. While the author does not specify the other boys' race(s), they are dark while Jerry is fair. They are apparently native to the area, while the white child is a tourist or foreigner.
Lessing grew up in a highly segregated colonial society in which children might have sometimes played together but were discouraged from forming friendships. As adults, many Europeans and most official policies promoted white superiority and anti-black discrimination.
Jerry initially wants to hang out with the dark boys, but they are older and make fun of him. He sees their abilities as natural because they are bigger and stronger. Rather than interact with them further and seek their guidance to master the necessary swimming techniques, he isolates himself from them. In contrast to what he sees as their natural abilities, he uses technology (the goggles), intellect, and hard work, analyzing the task and persevering to develop the necessary skills.
Jerry's successful swim through the tunnel thus symbolizes the white Europeans's conquest of an alien environment through skill. His coming of age coincides with his rejection of social engagement with the dark natives, symbolizing the white colonists segregated policies. Once he has conquered this hostile natural environment, like other colonizers, he loses interest and will, we infer, seek other lands to conquer.