There are various possible endings in Wadanohara and the Great Blue Sea.
This is the "True End". Listed is how this End is reached and what happens in this End.
To get to this ending, one must have played The Red Sea Witch ending (the first Normal End) and The Blue Sea Witch ending (the second Normal End)first.
Wadanohara finds Sal and an injured Samekichi. When the player talks to Samekichi, a cutscene starts. Sal gives Wadanohara an option to trust Sal to save the sea and Samekichi. In the third playthrough of this cutscene, the player will have the option to.. do nothing (which is clearly the obvious choice in a situation like that).
If that option is chosen, Wadanohara will question Sal on why he did the things he did. Sal will respond that they were both doing what they thought was right. Sal then quickly dashes forward and stabs Wadanohara through the chest with the Sacred Sword, before retreating into the true Sea of Death. Samekichi drinks some of Wadanohara's blood (which gives familiars great power), and begins to follows him in, telling Wadanohara that he will find the sword and return the sea to normal, but Wadanohara warns him that he will be sealed away in the Sea of Death forever. Samekichi informs her that he doesn't care about the sea but only wishes to make her happy, before charging through the portal to the Sea of Death.
The ending sequence starts with the recovering Wadanohara going to the boat to be alone, where she admits her feelings for Samekichi and start blaming herself for his disappearance. Fukami then approaches her, and comforts her, telling her it's not her fault as Wadanohara cries into his arms. Shortly after, the postman approaches her, telling her he found her ocarina floating in the water and knew it was hers as he originally got it for Samekichi to give to her. Wadanohara then starts to play it, telling herself she'll stay strong and await Samekichi's return.
It then returns to the "Sea Witch" concluding the tale and inviting the listener (the player) to watch the full moon with her. They return to the surface, where under the full moon, Samekichi returns and embraces the Sea Witch, revealed to be Wadanohara as it fades to white.