In the beginning of the story, we follow Cui Ying's POV, about her English teacher and Cui Ying's first love, that I think their relationship are connected to each other.

We still follow Cui Ying's POV from the beginning until the part:

Everything was silent.

She couldn't feel anything.

That POV actually still Cui Ying. But the interesting part, that memory of drowning overlap with someone else's memory.

The author tried to bring us to another person's POV, guess who? Cui Ying's English teacher, Li Xuan! We didn't get her name in Cui Ying's POV.

In part 6, we continue with Li Xuan's POV, when she adopted a child, who is a fellow survivor of the sinked boat accident made by the cultists.

Guess who the child Li Xuan adopted is? Yup, that boy is Cui Ying's first love!

In my opinion, from the beginning, the author kind of hinted that Li Xuan (the English teacher) is the adoptive mother of the boy (Cui Ying's first love). How they move to Cui Ying's school probably at the same year, the resemblance on how they dressed, and in the end, the author just told us clearly about it.

It's sad that the boy experienced tragedy involving water accidents. First from his childhood, his mother left him. And the second in his highschool graduation trip, his love (Cui Ying) left him. T^T