From isis.dreamwidth.org: "On the Japanese-flavored island of Mizukuni, members of the royal family who remain unmarried on their 21st birthday must ritually cast a ring into the sea and reel it in, re-enacting the mythic beginnings of their kingdom."
From fail-fandomanon.dreamwidth.org: "Unwed at 21, prince Shizuka is offered up the sea as a symbolic bridegroom in a traditional ceremony. But then, surprising everyone, a god of the sea called Kisui decides to take up the offer. Quite funny and cute. Contains deflowering and mermen."
From delicious.com: "Shizuka is a young prince, but as he's reached his 21st birthday without getting married it is time for him to undergo the ancient courtship ritual with the sea gods. Of course because it's so ancient, no one was actually expecting a merman to accept the ring he throws into the sea..."
From delicious.com: "A prince somehow manages to catch a fish...man in an ancestral marriage ceremony. The king thinks it's all a ruse to bone a dude and the queen is super worried he'll anger the Fish God by being a bad lay. Super fun!"