I think she killed them all. I think the ship represents the boat taking the dead. Think about it, only five people survived the crashland, five guys: Inoue, Sasaki, Kakuta, Fukuyama, and Sugihara (MC). no mention of a girl.
This looks like shoujo-ai not yuri but shoujo-ai, a lot of subtext especially that kiss.
Oh good, I'd been hoping to see this here. The latest chapter bringing us some actual yuri instead of just subtext helps, too.
in the later chapters there are moment of outrageous yuri (and also very disturbing dark moments). I am torn between thinking Gaku is either a psychopathic yandere manipulative lesbian lolicon, or an obsessive lesbian in denial.