William learnt to swim at the age of 2, and could freedive to 15m at 8 years old, but didn't begin serious training for the sport until 2003. Initially he studied in Italy under 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, in April 2007, he broke his first world record in the discipline of CNF (Constant Weight No Fins), diving to 81m.

Since then he has broken this record multiple times, and 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 Dolphins.

In July 2016 he furthered this record to 102m (334 feet) as part of the Steinlager Born to Defy project, broadcast live to NZ television.

William has also broken the world record multiple times in the Free Immersion discipline, most recently with 124m (406 feet) at Vertical Blue, in May 2016.

In 2019 he became the first person to complete an 'underwater crossing' of a major channel, swimming only underwater over the course of 934 breath holds to cross the Cook Strait between New Zealand's North and South Islands in 9 hours 15 minutes.