The drama is inspired by a mermaid tale found in Eou Yadam, the first collection of unofficial historical tales from the Joseon era.

Filming began on August 19, 2016, in Goesan, North Chungcheong Province, South Korea, before the production team flew to Palau to shoot underwater scenes.

On September 11, 2016, cast and crew traveled to Spain to film in locations such as A Coruña, Ribadeo, Lugo, Tossa de Mar, Castell de Santa Florentina in Canet de Mar, Begur, Sitges, and Barcelona, across Catalonia and Galicia.

Due to the busy schedules of the actors, the first script reading was not held until October 8, 2016, two months after filming had already begun, and it lasted three hours covering the first four episodes.

The drama reunited Lee Min-ho and Kim Sung-ryung, who had played mother and son in the 2013 hit drama The Heirs.

Lee Ji-hoon and Shin Hye-sun, who play stepbrothers-in-law-adjacent characters here, had debuted together in the 2012 school drama School 2013.

Jun Ji-hyun, Hong Jin-kyung, and Na Young-hee had previously worked together on the 2013 hit drama My Love from the Star before reuniting in this series.

The name given to Jun Ji-hyun's mermaid character, Shim Cheong, means 'extremely stupid' in Korean (combining 'Shim' for 'extreme' and a shortened form of the word for 'stupid'), while also coincidentally sharing its name with a classic Korean folklore character.

The series opened with a strong nationwide rating of 16.4%, surpassed the 20% mark in the Seoul metropolitan area by its sixth episode, and maintained the top ranking in its timeslot throughout its run.

After the broadcast run, a director's cut of the first two episodes with additional scenes was released and was well received, with many feeling these versions were better than the original broadcast cuts.