All of the footage at the beginning featuring a large ocean liner striking an iceberg and sinking is stock footage from the British Titanic movie Atlantique, latitude 41° (1958).
A sixth movie was contracted, but due to this movie's dismal box-office performance, and even worse critical reaction, it was swiftly cancelled.
The dam bursting scene is footage taken from the Dirk Bogarde movie La vallée de l'or noir (1957). Bogarde is in the green checked shirt and Stanley Baker in the red shirt; both are recognizable in this footage.