He's held the world record 15 times and was the first person to break the 100 metre barrier unassisted, truly dominating his sport.
To set these world records, William has the ability to hold his breath for 8 minutes underwater, pushing his body both mentally and physically.
He's just published his memoir, titled Oxygen and joined Jack Tame to talk about his extraordinary lifestyle. 
LISTEN TO THE FULL INTERVIEW WITH JACK TAME ABOVE