Leviathan

Leviathan, also known as Deep Sea Water is an ongoing Korean post-apocalypse manhwa written by Lee Gyuntak and illustrated by Noh Miyoung. It is published by Toomics, which also handles its English release.

Taking place on a post-apocalyptic aquatic earth a century after a calamity rose the sea levels, Leviathan follows the adventures of siblings Bota and Rita after their father, Teon Ma, sacrificed his life to save them from a sea monster. The two are soon taken in by a woman called Kana, an acquaintance of their deceased father.

Synopsis
The world is submerged, and sea monsters rule the earth.

Will Bota and his family ever make it to safety and escape this aquatic hell? ...And who in the world is Kana?!