| Quote #1
Once we were dolphins and Imo made us into men! [...] If that wasn't true, then there was just dark water and nothing was anything... (3.117)
Mau is clinging to the last vestiges of his faith here. The promise of an afterlife is a critical component to most religions. Darkness and nothingness are the most scary things of all, and religion (usually) offers some version of light and warmth afterlife.
| Quote #2
Would [Imo] have made useless gods? There was, out of the darkness inside, another thought that [Mau] wouldn't even have known how to think a few days ago. (4.89)
Surprise, surprise: Mau's doubting the gods again, and that's scary. Mau's religion doesn't encourage contrary thoughts, so even the fact that he's questioning his faith makes him think these are "dark" thoughts.
| Quote #3
The Nation was blessed, people said. (4.104)
Talk about an ironic statement. The Nation was swept away by a deadly, violent tsunami. If that's "blessed," we'll take … unblessed. (Except that Mau ends up seeing the tsunami as a good thing—for him.)