William learnt to swim at the age of 18 months, and was freediving to 15m by the age of 8, however he did not begin serious training for the sport until 2003. In 2004 he became the first non-Italian instructor at Umberto Pelizzari's prestigious Apnea Academy. Since then he has trained extensively with Pelizzari, and written the translations of 'Manual of Freediving' (Idelson & Gnocchi) and the Apnea Academy Instructors Manual.

William holds the world record in the discipline of Unassisted Constant Weight (no fins), and can dive to 95 meters (288 feet) without the use of fins, rope, weight, or any other form of assistance. In 2010 he also broke the world record in the discipline Free Immersion, with 116m (380 feet). He currently trains in Tenerife, at Tenerife Top Training Center during the summer and Dean's Blue Hole in the winter.

Personal Bests in competition:
◊  CNF (depth with no fins)          95 meters (311 feet)
◊  CWT (depth with monofin)       116 meters (380 feet)
◊  FIM (depth pulling on line)       116 meters (380 feet)
◊  DNF (distance with no fins)      174 meters
◊  DYN (distance with monofin)    237 meters
◊  STA (stationary breath hold)    7 minutes 29 seconds