The espionage elements added to the film are inspired by the fact that the author of the book, Arthur Ransome, had actually worked for British Intelligence, spying on the Russians. The code name "S76" that appears in the film was Arthur Ransome's actual code name.
The book's character, Titty, was based on a real girl who used it as a nickname. She got the nickname from the poem about "Titty and Tatty Mouse". The filmmakers decided to change the name to the other mouse.
The opening journey was filmed on the Keighley and Worth Valley railway. The same line used in The Railway Children.
Dan Travers, who was the diving and marine supervisor on the film, is the son of Virginia McKenna, who played Mrs Walker in the 1974 film of Swallows and Amazons.
Based on the book of the same name by Arthur Ransome, published in 1930 and set in the same location of the Lake District in the summer of 1929. The book does not have a spy story within it, although 'Captain Flint' is still the uncle of the Blacketts and does live on a houseboat.