Earlier quoted context omitted.
When swimming underwater, brain should not be telling or ignoring anything, it should be totally empty. Idle brain consumes about 25% oxygen, even more when it is being used.
I would love to see a source for this that thinking about something uses more oxygen.
I got my open water scuba certification many years ago from a former Navy SEAL.
At the time, I was 4' 8" and weighed 85 pounds.
The Navy SEAL was about 5' 10", 200 pounds.
Every single dive we went on he used less than half the air that I used, and I used less air than the rest of the dive group (because I didn't need as much given my stature).
He explained that he used a meditative style breathing pattern and simply remained very calm underwater.
/anecdata