During world war two, German U-boats used to have issues with chlorine gas being produced from the batteries and it would flood the boat and kill everyone onboard.
They would also bring up proof, usually something that could be tied to the vessel, which could be a piece of military-grade equipment or even silverware engraved with the names of the ship or the enemy seal.
This is what the suits looked like:
My grandfather was a submariner in the Pacific during the war, they were even inside Tokyo bay during wartime. He had many stories, one of them was that the batteries always got the priority in regards to water. Especially on patrol and missions, they would often go days between wiping themselves down with hand towels from a basin of shared water, all to preserve the fresh water for the batteries.
He was on the Saury
When I replaced the AGM battery on my German car, I learned that, even though they don't vent under normal conditions, still have a vent hole. But that's paired with a pressure regulator and not for normal conditions.
Which makes me wonder: did BMW start to use AGM so they can move the heavy battery to the trunk, which helps with weight balance? Or was it an emissions thing that enabled them to move it to the trunk.
I believe traditionally, a diver will bring up a ship's bell for this purpose, if they are able. But maybe I'm off base about about, particularly when it comes to an enemy ship.
https://en.wikipedia.org/wiki/Ship%27s_bell
https://www.washingtonpost.com/history/2024/02/20/bell-ship-....