After the world surface water has become thin quicksand people live on small islands in this sand sea, or sakai. These pockets of humanity try to survive by hunting Leviathans (mutated whales), while avoiding Devourers (mutated sharks).
Leviathans are now the dominant species on the planet and viciously attack humans if able.
Some of the fun of this manga and recognising things from science fiction movies and shows appearing in the background, as the author has admitted that this is a tribute to the world of science fiction.
Good for a distraction, reading this won't take long.