Director Jack Arnold liked to use the sides of the movie screen as the arches in a proscenium-style theater with unexpected intrusions coming in from the sidelines. This technique can be seen here when 'John Agar (I)' goes out into the darkened area around the motel to look for Lori Nelson's missing dog. A hand suddenly reaches out from the right side of the screen to touch him, giving him (and the audience) a start, but it's not the dreaded Gill Man. It's simply Lori Nelson.
For this film, Ricou Browning wore a creature head intended for John Wayan, who had originally been hired to play the creature in this sequel. The creature suit had to be cut down to fit Browning.
Look for a young, uncredited Clint Eastwood in his first screen appearance as the goofy white coated lab assistant who does the silly mouse gag in the lab scene with the monkey.
Reported to be the highest-grossing film of the "Creature" trilogy.
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, because his Gill-Man costume had become waterlogged and was too heavy for him to fight the current that was pulling them down. 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.
The only one of the three Gill-Man movies to be parodied on Mystery Science Theater 3000.
This film is the only sequel to a 3-D film to be shot in 3-D as well.