The author describes a stream of water flowing. “S. finds himself irritated by how effortlessly water finds the place it belongs.”
This highlights S.’s conflict with identity, while drawing from the extended metaphor of water and alluding to the opening quote “What begins at the water ends there, and what ends there shall once more begin.”

After losing Pfeifer, “They were five, and now they are two. Two people with one light, scrambling through twisting passages that are becoming more and more steeply downsloped.”
Eric says “Diminishing numbers – like the ship.”
One could also note connect this to the diminishing number of members in the S.

Jen and Eric get into an argument because he didn’t tell her about the steam tunnels that he uses for travel.

“You should give me a copy of your map. Just in case. It’s in both of our interests.”

The concept of traveling in secret highlights their constant paranoia.