The Blue Sea is the setting of Wadanohara and the Great Blue Sea, the sea where the protagonist Wadanohara was born, and where the entirety of the story takes place. It is currently ruled by Uomihime and is located in the Pitch Black World.
The Blue Sea was once ruled by The Sea King. After entrusting the Blue Sea to his eldest daughter, Uomihime, he fell into a deep sleep. Not long after, Mikotsuhime, the Sea King's younger daughter, rejected her father's decision to appoint Uomihime as his heir. She wreaked havoc and threw the Blue Sea into chaos, declaring her desire to be ruler instead of her sister. In turn, she created her own kingdom of sorts, the Sea of Death, in order to seize control of the Blue Sea. The quarrel went on for years, until Wadanohara's father, Meikai, sacrificed his life to end the war. After Meikai's death, the Blue Sea fell back to peace, with Uomihime as the ruler.