The trained dolphins who played Alpha and Beta were named Buck (for screenwriter Buck Henry) and Ginger (for dancer Ginger Rogers). On the next to the last day of filming, when their parts were done, they escaped and never returned.
Roman Polanski was originally supposed to direct the movie. He was in England scouting locations when his wife Sharon Tate was killed by the Manson Family.
The name of the small boat which transports George C. Scott to and from the mainland is "Erewhon II." It's a reference to the 19th-century satirical novel "Erewhon," by Samuel Butler, about a society cut off from the rest of the world which proves to be less of a paradise than it seems. "Erewhon" is an anagram of "Nowhere."
George C Scott was paid $750k.