The Sea God is thought to be Uroko-sama, however, Uroko-sama is only a scale of the Sea God. The Sea God, in the anime, is shown to be a flame who watches over the sea and keeps Ohshioshio lit.
The people of Ohshioshio are descendants of the Sea God and from Ojoshi-sama, who was a human.Manaka, Hikari, Kaname, Chisaki and Tsugumu are all protaganists of the first part and the second part adds Miuna and Sayu as protagonists.
Ojoshi-sama was a human girl, who could live in the Sea and whom the Sea God fell in love with. They had two children. Ojoshi-sama loved someone from the surface, the Sea God knew the fate of her beloved (death) and took away her feelings as well as leaving her on the surface without memories of him. It was revealed at the end, that she, despite loving someone else, also treasured his feelings and their children. Which made the Sea God realize what he has done and burn away the Ojoshi-sama's graveyard.
Ojoshi-sama had a similar look as Miuna, having black hair and greyish eyes as well.
• Miuna looks similar to her, in the preview of Episode 26, she was mistaken for Miuna.
• Looking closer to the scenes with her children, they just look like Manaka and Hikari.