Actor/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.
Much of Revenge Of The Creature was filmed on location at the Marineland of Florida oceanarium. The actors 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.
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 Lamb, who had originally been hired to assist Tom Hennesy in playing The Creature in this sequel. Lamb was dismissed after filming began, and Browning was hired to replace him. Lamb's Creature suit had to be cut down to fit Browning.
Tom Hennesy is The Creature in the receiving tank fighting John Bromfield and others, when a cargo net is dropped on him to control him. In the next scene when The Creature is being escorted into the permanent tank underwater, The Creature in the net is Ricou Browning, and Tom Hennesy is the diver swimming next to him wearing white swim trunks. Later Hennesy can be seen as the first diver climbing out of the tank.