Colin Firth films at sea for true-life The Mercy

Marsh and his team filmed scenes in Teignmouth on the Devon coast, from where Crowhurst set sail in October 1968, but ocean-set sequences were shot 75 miles further east, off the Isle of Portland, where the crew fought the tides and bad weather.

“I think Portland and Weymouth were the biggest challenges because of the wind, changing weather and waves,” says producer Peter Czernin.

Ocean filming also took place in the Mediterranean off the coast of Malta, where the team faced daily challenges keeping the coastline out of shot, while also having to head further out to sea than they would otherwise have planned in order to steer clear of other boats.

“We were shooting on film so magazines would run out while we were out there so we had all the logistics associated with that,” says Czernin.

“Then there’s the fact that the crew need to eat and go to the loo. It was kind of bonkers and very difficult. I don’t think I’ll make another film on the water in a hurry.”

For the Maltese leg of the shoot, the production team was restricted to just eight people – as opposed to the usual 30 – on a main crew boat, while the camera department and hair and make-up each had a boat to themselves. There was a separate boat for Marsh and his immediate team, a boat for the runners, several safety boats and an additional larger vessel that had toilet facilities and drinking water.

“I can see why people want to shoot films in the controlled environment of a tank where you can very easily control the movement of the boat,” says Marsh. “But the actual motion of the boat and the experience of shooting with Colin on the boat was so important to the texture of the film.”

Ocean-set films are more commonly using land-based sets with complex, computer-controlled rigs or visual effects to simulate the movement of the sea.

Both the most recent Pirates of the Caribbean movie and the upcoming superhero film Aquaman – much of which is set underwater – used variations on these techniques to ensure complete control for their respective production teams.

Aside from the training and safety protocols needed to have actors and crew in water, sets rapidly degrade when in contact with the chemicals needed to keep the water clean, an additional production challenge producers would rather avoid.