Diving into the Deep, Becoming the Overlord

Two hundred years later, Earth is completely submerged by global seawater, and humanity is forced to survive in makeshift floating slums on the ocean. Kyle, a boy from the lowest rungs of society, dives into dangerous underwater ruins to save his sick sister. There, he accidentally awakens a lost nuclear submarine from the old era. The submarine recognizes him as its captain. He is then hunted by the Iron Hook warlord, who monopolizes water sources and rules with brutal tyranny. As Kyle unlocks the submarine's combat capabilities, he ventures through deep-sea forbidden zones, suffers crushing despair, and ultimately awakens to become stronger.