The 54-year-old actor has been learning to sail in preparation for his role as yachtsman Donald Crowhurst, reported New York Post.
Firth revealed the news while attending a yacht party at the ongoing Cannes Film Festival.
The film will follow Crowhurst's disastrous attempt to win the 1968 Golden Globe Race, which ended with the yachtsman inventing an outrageous account of travelling the world alone by sea.
It's being directed by James Marsh, who helmed the Stephen Hawking biopic "The Theory Of Everything", and Rachel Weisz has also been linked to the film after Kate Winslet reportedly pulled out of the role of Crowhurst's wife.