The submarine used for filming is called the Black Widow. It is an old Soviet sub moored in the River Medway at Strood, Kent.
When the crew are approaching the submarine for the first time in the port of Sevastopol, they pass the moored Russian missile cruiser Moskva, with it's hull number 121 clearly visible. This ship would later be sunk by Ukrainian anti-ship missiles on 14th April 2022
Jude law spent 3 days on a British submarine to prepare for his role.
The small Canon EOS C500 made the shots in Ukraine possible according to director Kevin Macdonald: "We could use the C500 handheld. A lot of the film is handheld. But we also used it on a dolly and on the crane, in one or two instances. So it was perfectly easy and flexible to use in that regard. There are a few shots in this film that Christopher Ross and I took on a 'recce'. We went to Ukraine; to Sebastopol, which is the Russian naval base that's in the Black Sea. We just really went to have a look, because we were hoping to go back and shoot there for real. But we ran out of money and weren't able to go back. Luckily Chris and I had taken the Canon C500... really just because we wanted to have a play around with it, as an opportunity to test it together. We took some shots out of the window of a bus and some landscape shots, which actually we've used in the final film. Chris was operating it on his own - he found that relatively straightforward, even with the Codex 4K recorder. It does eat up memory like you wouldn't believe but we shot some really beautiful images on the Canon. So that's one of the great advantages. It's a camera that you can take along on a 'recce' with you; a camera which can just sit in a backpack and, if you suddenly see something and think 'ah that's fantastic; I'd like that in the film. I'm never going to see that again' - a sunset, an animal or whatever it is - you can whip it out and there you go; you've got it."
In the sunken U-boat, there is one body with a bullet hole to the head and holding a pistol. The gun is a Luger Parabellum P38, which marks the body, along with the hat, as the sub's captain.