William learnt to swim at the age of 18 months, and was freediving to 15m by the age of 8, but didn't begin serious training for the sport until 2003. Initially he trained in Italy with Umberto Pelizzari and other members of the Italian Apnea Academy, helping to found Apnea Academy International with English translations of the texts.
In 2005 William was the first freediver to dive at Dean's Blue Hole, now recognised as the world's premier freediving venue, and site of the annual Vertical Blue event. There he broke his first world record in the discipline of CNF (Constant Weight No Fins) in April 2007, diving to 81m. Since then he has broken this record multiple times. In 2010 he became the first human to descend to 100m (330 feet) during Project Hector, an event aimed at bringing awareness to the plight of New Zealand's critically endangered Hector's and Maui's Dolphin species.
In July 2016 he furthered this record to 102m (334 feet) as part of the Steinlager "Born to Defy" project, which was broadcast live to New Zealand television.
William also holds the world record in Free Immersion, with 124m (406 feet), set at Vertical Blue in May 2016.