The initial ones could come from the lungs of the person. When you do a free dive you pump your lungs full of air and gradually release that air over the cours

The initial ones could come from the lungs of the person. When you do a free dive you pump your lungs full of air and gradually release that air over the course of the dive. A trained swimmer can easily swim without breathing for 50 meters, producing air bubbles all the time. (EDIT: active swimming, aiming for distance. I had to do this when I got my lifeguard certification - IIRC 25m sufficed to qualify, but everyone did 50m). That video is unfortunately way too short to prove anything.