The series was originally developed under the title 'The Deep', with Freeform ordering a pilot on July 25, 2016 based on a story by executive producers Eric Wald and Dean White, with Wald writing the script and Scott Stewart directing.

Eline Powell's character was originally named 'Po' when she was cast on August 24, 2016, before being renamed Ryn for the series.

The main cast was assembled gradually in 2016, with Rena Owen cast as Helen alongside Powell, followed by Ian Verdun as Xander in September 2016, and Alex Roe and Fola Evans-Akingbola joining shortly after as Ben and Maddie.

The pilot was filmed in Vancouver, British Columbia, Canada in October 2016, and full production on the remaining episodes ran from August 4 to November 22, 2017.

Filming for the third season took place between August 1 and December 16, 2019.

Composer Michael A. Levine wrote the haunting 'Siren Call' theme, performed by singer-songwriter Mariana Barreto, incorporating unusual instruments like the tenor violin, octave viola (ciola), and the gusli.

The first season received critical acclaim, holding a 95% approval rating on Rotten Tomatoes based on 20 reviews, with critics praising its unique and empathetic take on mermaid mythology.

The first episode premiered as a preview at New York Comic Con in October 2017 before the official broadcast in March 2018.