When I go for a horror manga that deals with survival, I expect:
2. A satisfying conclusion
3. Violence and blood (Optional but appreciated, not a dealbreaker)
I for one, am a sucker for suspense. If there's an inkling of it, I'm in and am probably all caught up in it while others are shaking their heads. This has done it for me in the suspense department and it had enough to make me want to continue on in one sitting. You got your mystery to go along with it; the disappearances, the lack of trust and a sense of something deeper. There's always a reason for everything and it explains it all. I may not agree or like all the explanations or answers, but at least it leaves no stone unturned. And while I may not like how all the events turn out, it still does them well enough overall.
It has it's share of blood; not overly detailed in it's gore but it's there for effect. Some violence is there to add some spice to it and it works some magic for the survival on an island theme. To go with your blood and suspense, is some fan-service with the buxom health/phys-ed teacher Yukino. Some nudity here and there, shower scenes, the ocean and more fan-service, though some is in the "your milage may vary" category.
One never gives away details of the ending so I'll say that it didn't work for me. If it were more grim, more like the setting, then sure, I would have liked it and rated it higher. Even without a satisfying conclusion, I still had a good time. There is better fare in this genre and setting but fast readers can blaze through the four volumes before the night ends.