Actor and stuntman Tom Hennesy almost drowned during filming. Playing the Creature, he grabs Helen Dobson (actually stuntwoman Ginger Stanley) on a pier and jumps with her into the water. The scene was shot at night, and when Hennesy and Stanley hit the water, they discovered it was full of jellyfish. In addition, a freak current started to pull them both down. Hennesy let go of Stanley, who swam to the surface, but Hennesy's inflexible Gill-Man costume had become waterlogged and too heavy to fight the current. He was rescued by two local boys who happened to be watching the filming from a nearby boat, and quickly raced over and pulled him in.
A young Clint Eastwood makes his first uncredited screen appearance as lab technician Jennings. He discusses with Professor Ferguson (John Agar) about an experiment involving a cat and four rats sharing the same cage. He points out that one of the rats in the cage is missing and accuses the cat of eating it, but discovers that the missing rat was in his lab coat pocket.
Much of this movie was filmed on-location at the Marineland of Florida oceanarium. The actors and actresses filmed in the water during the tank scenes were required to swim with its real-life underwater inhabitants, including sharks, eels, barracudas, and more. Despite the close proximity to ocean predators, the only incident was that of a sea turtle biting off the prosthetic foot of Ricou Browning's Creature costume.
Many of the extras in this movie were employees at Independent Life Insurance Company in Jacksonville, Florida.
At the beginning of Retour vers le futur 3 (1990), Marty exits a movie theater dressed in his cowboy outfit, and tells Doc, "I'm pretty sure Clint Eastwood wouldn't wear this." When he says this, he is standing next to the poster of this movie, which is Eastwood's screen debut.