Daryl Hannah, a vegetarian, refused to eat real lobster for the restaurant scene. The crew scooped out the insides of real, cooked lobsters and filled them with a thick, tofu-like paste. Ron Howard said Hannah cried after each take over the deaths of the lobsters for their shells.
The "Crazy Eddie" commercial that surprises Madison was for a real electronics store. Eddie and Sam M. Antar opened Crazy Eddie in Brooklyn, New York in 1971. Their spokesman was WPIX-FM disc jockey Jerry "Dr. Jerry" Carroll, whose frenetic nonstop sales pitch was based on used car salesman Earl "Madman" Muntz. The pitch always ended with "Crazy Eddie, his prices are IN-SA-A-A-A-A-A-ANE!" The chain grew to 43 stores in four states. It closed in 1989 after charges of fraud and security violations.
Daryl Hannah had been swimming "mermaid" style with her legs bound together since she was a child, due to her fascination with Hans Christian Andersen's "The Little Mermaid" story. In the documentary contained on the 20th Anniversary Splash! DVD, Tom Hanks recalled how the other cast members would drop French fries over the side of the tank to her as though she were a trained sea mammal, for she couldn't leave the water while her legs were "shrink-wrapped."
While Allen is offering girls' names to the mermaid before she settled on Madison, the last two he suggests are Elizabeth and Samantha. Elizabeth Hanks is Tom Hanks's daughter and Samantha Lewes was his then-wife.
The map from the shipwreck that Madison uses to find Allen's home is an old map of the Province of New York. It bears the name 'His Excellency William Tryon Esq.' Tyron was the colonial governor of the Province of New York from 1771 to 1780.
Daryl Hannah's fin weighed 35 pounds. It took technicians three hours per day to put it on her, who had to remain still while it was being attached. "At lunch they'd yank me out on a crane and plop me on the deck," she told People. "I couldn't eat because I couldn't go to the bathroom. I just lay there shivering with barnacles in my hair, soaking wet."
In the scene that takes place just before a friend's wedding, Allen yells at a guy, "That's the news, ya want the weather?" Then Freddie tells him that's the bride's brother. The actor was Ron Howard's brother, Clint Howard.
The scene where a television show makes Madison upset, and Allen explains the television show is fictional, and didn't really happen was originally going to set up a joke later on where Madison watches a real life plane crash on the news and laughs.
The film was rushed into production to compete with a similar film called Mermaid about a mermaid starring Warren Beatty and written by Robert Towne. A possible actors' strike delayed progress, and eventually the project was declared dead.
The film playing on television in Bloomingdales, was Disney's The Black Hole (1979), which was the first Disney movie to be rated PG. Splash was the first film by Touchstone Pictures, which was set up by Disney to cater to adult audiences.
Robert Short is credited as the man behind the mermaid costume. He initially pictured the mermaid with a dolphin-esque tail as it would make "biological sense" but Howard rejected the design insisting on a more tropical look. Short eventually created a tail based on koi fish. He would later admit that this was a genius idea since "you lose a lot of color underwater" the red tail stood out more.