During production in the Dominican Republic, Diana Nyad and Bonnie Stoll visited the set. Nyad was able to swim with Annette Bening, with "many of the 400-person crew cheering". She said, "That's a moment I will never forget."
To prepare for the film, Annette Bening worked with former Olympian Rada Owen for roughly a year, as well as a personal trainer in the gym.
Diana Nyad teases Johnny Carson about how many times she'd had to do the show before she finally met him. This is a reference to his famously reduced work schedule (three days a week, frequent vacations), during which guest hosts filled in.
Almost all the water seen is not real. For safety, budget and logistical reasons, the scenes in the sea were filmed in a tank, and by recreating the ocean in CG, the director could have creative input in what it had to look like. What is seen in the final version is a combination of the live-action shots with CG water.
An earlier script by Robert Specland was featured on the 2015 Black List of the best unproduced scripts of the year.