Cornelius Drebbel was the man who built the first navigable submarine in 1620 while working for the English Royal Navy. Using William Bourne's design from 1578, he manufactured a steerable submarine with a leather-covered wooden frame. Between 1620 and 1624 Drebbel successfully built and tested two more submarines, each one bigger than the last. The final (third) model had 6 oars and could carry 16 passengers. This model was demonstrated to King James I in person and several thousand Londoners. The submarine stayed submerged for three hours and could travel from Westminster to Greenwich and back, cruising at a depth of from 12 to 15 feet (4 to 5 metres). Drebbel even took James in this submarine on a test dive beneath the Thames, making James I the first monarch to travel underwater. This submarine was tested many times in the Thames, but it couldn't attract enough enthusiasm from the Admiralty and was never used in combat.
To re-oxygenate the air inside one or more of these submarines, he likely generated oxygen by heating nitre (potassium nitrate or sodium nitrate) in a metal pan to make it emit oxygen. That would also turn the nitrate into sodium or potassium oxide or hydroxide, which would tend to absorb carbon dioxide from the air around. That may explain how Drebbel's men were not affected by carbon dioxide build-up as much as would be expected. If so, he accidentally made a crude rebreather nearly three centuries before Fluess and Davis.