Survival

From MangaHelpers:

After a terrible earthquake, Satoru wakes up in a pitch black cave that him and his friends were exploring. Desperately moving through the crumbling cave with just his backpack, he expects a warm rescue once he gets out. But what awaits Satoru was an unbelievable fact: Japan has sunk underwater due to the earthquake and he is stranded on an island with no one to help him. Can Satoru learn how to survive by himself?

A classic survival manga by Takao Saito, the mangaka of Golgo 13. [ruggia]

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From JManga:

"Japan, a land in peril... A land broken by war, natural disaster, and terror. A young boy refuses to abandon the search for his family across the warring land of ruin. ""So, you managed to survive, did you?"" This story of survival reaches farther than anyone would imagine... A tale spanning the period separated from the young pup, Shiro, Satoru finds his life spared having been swept away by rushing river of mud and debris, only to wind up in the pit of ""The Empire"", ruled under the hand of a certain dictator... Can the action of Satoru alter the fate of the oppressed?"

Note: Most editions of Survival include a bonus volume called "Survival: Another Story."