In the years BW (before the wave), the culture of Nation required a rite of passage for boys reaching the age of thirteen. To acquire a man's soul and become a member of the Nation, a boy was left on Boys' Island, a smaller island not far away (an easy day's paddle from Nation). There he had to survive for a month alone, while making himself a canoe, and then return to Nation. This feat was not quite as onerous as it sounded because previous generations of boys and their fathers and uncles had left clues and tools, particularly an axe, to assist him. Boys were terrified of their blighted futures should they fail, but most managed the task, as did Mau, and when they returned there was a feast to welcome and acknowledge a new man.1
When Mau returned, however, there was no feast or celebration; there was no Nation, only wreckage and some corpses. Before he could return with his new soul his world vanished. The tsunami washed away the reason for the rite, along with Mau's belief in the gods who were supposed to provide the soul.
In the new Nation, several generations later, there is still a rite of passage for boys and girls. It only lasts one night while they stand guard on the beach as Mau did, to watch over their soul and see what they may see.
1 And the bit with the very sharp knife of course, but there's always a catch.