Harukanaru Toki no Naka de: Ryuuguu no Miko

The protagonist (Yuikawa Akari), is a college student in modern world and went on ferry trip with her friend. When the ferry is sailing over Dan no Ura, A storm came and countless of monster appeared from the sky.

A man with graceful kimono came to protect the protagonist from the monster, carrying her into the depth of the ocean
"I am Kiyomori, in the name of Heike clan I will guide the princess"

He guided the protagonist to the city of underwater called "Ryuuguu" , ruled by Ryuujin and chosen as its priestess. With the Hachiyou, who are the chosen heroes whose name engraved in history, the protagonist began her journey to save the spacetime.

[Translated from official website]