Gadget: Miniature Rebreather
Provides approximately four minutes of air in an emergency situation.
Bond uses it to escape from his investigation of the Disco Volante, after he disposes of his scuba tank, which floats to the surface and convinces Largo's thugs that he is dead. Later, Bond uses it to survive in Largo's pool/shark tank and during the climactic battle.
One way, or another, I'm gonna find ya...