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.
This island is filled with apple trees, grassy hillsides covered with flowers and sandy paths. During a flashback in WATGBS, Wadanohara and Samekichi are to go there at the request of Irena to collect apples.
- There is a sign here saying: "Caterpillars exterminated." This is a reference to the numerous amount of caterpillars in Apple Park in The Gray Garden.
- The treasure chests found here contain four Sunglasses and four Fu's.
Hydrangea Isle is filled with hydrangeas and snails. It appears to be always raining there. One of the six barrier stones is located on this island.
- Tsumuri makes a cameo appearance here, but the player is unable to interact with him.
- Usagi-san has made a cameo in the Wadanohara and the Great Blue Sea -Reboot-. They can be found in the back of Hydrangea Isle.
Lonely Isle is filled with black cross graves. One of the six barrier stones is located on this island. In WATGBS, Wadanohara and her familiars go there to fix the barrier. When going in the cave to fix the moonstone, they find an injured Samekichi and battle him.
- Noir makes a cameo appearance on this isle.
Rainbow Isle is filled with a variety of flowers and balloons. Wadanohara and her familiars go there after they had fixed all of the barrier stones. While on the island, they meet up with Chlomaki and strike a conversation.
- Visit Sea Kingdom for more information.
Star Isle is an isle whose surroundings and trees are covered in star fruit. Wadanohara and her familiars first visit it on their way back to the Sea Kingdom at the start of the game. This is also the place where the player meets Nekoyama for the first time.
During a flashback, this isle is the place where Samekichi gifted Wadanohara her ocarina. It is also the place where he first confessed his love to her, though she did not fully understand what he truly meant by this.
Windmill Isle is an island filled with Windmills. One of the six barrier stones is located here.