Maybe my media literacy skills are going down the drain or did anyone else not have a clue what was happening.. I have no idea what to make of this.
Update: Nevermind, I went into some forums and found some explanations. The “world ending” was never meant to be literal— which was already apparent to me and likely most other readers, but I couldn't quite make out what the end was supposed to mean. A lot of people speculate it was Yura’s way of expressing that she wanted her life to end. The sea is used as a metaphor for depression and drowning in despair. And all those little moments— like her injuries and strange comments, were warning signs Koma either didn’t pick up on or deliberately chose not to acknowledge— hoping to stay in her own comfortable bubble. By the end, it’s implied Yura took her own life, likely by drowning after inviting Koma to come and do it with her, and the final scenes are Koma looking back on those missed signs and being overwhelmed with guilt. I'm still not entirely sure if I'm interpreting it correctly or if I've understood everything, likely not, but this is what I could gather from those forum discussions.