Wadatsumi no Musume

There are four islands at the southern end of the world: Karoku, Kada, Uka, and Shamon. The lords of each island are to marry the "daughters of the Sea God," who are chosen by the oracle of the Sea God. By marrying these daughters, the islands will gain the blessings of the sea god and prosper.

By the Sea God's decree, Ran was destined to marry Kei, the young lord of Karoku. Yet, her father had been executed by Eikun, the previous lord of Karoku. Filled with unease at her fate—marrying the son of the man who killed her own family—Ran sets out toward her wedding ceremony in a small boat under the moonlight...

(Source: Kodansha, translated)