Simhaesu (2018)

Blurb:
“The Survival of Humanity in a Submerged Earth” The endless monsoon began when an icy comet collided with the Moon, triggering a catastrophic flood on Earth. For 70 years, unrelenting rain fell, ultimately submerging the entire planet. Having lost their land, humanity’s survival now depends on life at sea. The once-dominant species that ruled the Earth is no more. Sea creatures, mutated into grotesque beings known as “leviathans,” now pose a deadly threat to human life. As the world teeters on the brink of collapse, two siblings, Bota and Rita, must navigate a dangerous new reality filled with monstrous threats, human conflict, and the destruction of the environment.

Source: www.anisearch.com/manga/56862