Oliver Reed got an ear infection and had to be hospitalized after participating in a sequence that was shot in the filthy waters of a river.
Sammy Davis Jr. visited the set during the shooting of the film.
According to Christopher Lee, the pond fording at Black Park was a horrible experience. The water was polluted and the bottom consisted of three or four feet of mud, sludge, broken trees and branches, stench, and general filth that the stunt men refused to do it. Michael Ripper nearly drowned, Oliver Reed contracted an eye and ear infection, and Lee claims he couldn't walk upstairs for nearly six months because of the after-effects.
The film was originally given an 'X' certificate for its cinema release by UK censor John Trevelyan. Hammer then decided they wanted an 'A' certificate so the film was trimmed of violence with heavy cuts to the killing of Maggie by piranhas. After reconsidering again Hammer decided to settle for a 'U' certificate so further cuts were made which edited all the piranha scenes, the blindfolded sword-fight, the attack on the farm, a shot of a hanged man's legs, and the overdubbing of words such as 'harlot' and 'adulterer'.
The penal colony where Jonathon Standing (Kerwin Mathews) was sent to was also the Callow Hill Location. The scene of his escape and his attempted killing by two men was shot in one of the swamps at Burnham Beeches. Other scenes of the pirates wading through the swamp and walking in wooded areas was a combination of Burnham Beeches and Black Park.
When approached by Michael Carreras to write a script for a pirate movie, Jimmy Sangster was cautioned by the producer that they couldn't afford a ship. The ship seen from long shot in the beginning was a stock shot.