Drama series will be the last acting role for actor Lee Min Ho before he enters into the military for approximately 2 years.
Krystal Jung and Kim Sung Ryung played cameo roles who previously starred together with Lee Min Ho in Sangsogjadeul (2013).
Filming began on August 19, 2016 in Goesan, North Chungcheong Province, South Korea. The production team then flew to Palau to film underwater scenes. The cast and crew flew to Spain on September 11 where they filmed in A Coruña, Ribadeo, Lugo, Tossa de Mar and Castell de Santa Florentinain Canet de Mar, as well as Begur (Es Cau Swimming Pool), Sitges (Town Hall and other backdrops) and Barcelona (exteriors of Palau de la Música Catalana, etc.), among other places in Catalonia and Galicia (Spain). The first script reading was held on October 8, 2016, two months after production began, due to the busy schedules of the actors. It lasted three hours and covered the first four episodes.
The story is inspired by a historical tale about a mermaid contained in a short story anthology known as Eou Yadam by Joseon-era scholar Yu Mong In(Yoo Mong In).
This Drama series is also the first acting role for actress Jun Ji Hyun since she gave birth to her first child.