Underwater breather

---- Issued to Bond, played by Sean Connery in the Bahamas by Q, the prop is famously used by 007 to infiltrate Largo's lair through the shark infested pool.
Utterly convincing on screen the prop was simply held in Connery's mouth while he held his breath.
The prop was created by the EON prop team at Pinewood studios in England, manufactured from a mixture of custom-made machined parts and some pre-existing items, including the micro-sized gas canisters that were used to carbonate the contents of a soda siphon, a staple in most 1960’s British households before the advent of widely available prepackaged soda. Often referred to as a rebreather, this prop is never actually named in the film.
Q simply says 'In the event of a rebreather not being available you can use this device'. ----- (jamesbond.fandom.com)