Nocker was racing in Friday's Gabon Grand Prix, round two of the Class 1 World Powerboat Championship, when he crashed on the fourth lap. The 47-year-old was rushed to Libreville's Omar Bongo Ondimba Military Hospital but doctors were unable to save him.
His Norwegian co-pilot Kurt Olsen remains in hospital in a critical condition with multiple fractures of the ribs and scalpula.
Eye witnesses described how Nocker's boat flipped and barrel-rolled after hitting two large waves in quick succession during the race.
"We are all extremely saddened by this tragic racing accident," said Niccolo di San Germano, president of race organisers H20 racing.
"There are very few suitable words at a time such as this but we all send our heartfelt condolences to William's family."
Fellow competitors paid tribute to Nocker by motoring out to the scene of the accident and holding a one-minute silence.
Nocker had only recently returned to competition after completing a five-year ban for tampering with an opponent's boat in 2007. Nocker's team-mate at the time, former European champion Rob Lister, was caught on CCTV pouring water into a fellow competitor's fuel tank before a race and both he and Nocker were banned for gross misconduct and unsportsmanlike behaviour.