Survival (Takao Saito)

Survival (サバイバル) is a manga written and illustrated by Takao Saito.

Plot
An inexplicable catastrophe plunges Japan to the bottom of the ocean. A boy survives the misfortune, but he has been abandoned in a land surrounded by sea on all sides. His effort to stay alive compels him to sharpen his wit, as he finds that nothing he learned in school is useful for staying alive.