Driver Jeffrey Tillman and throttleman Robert M Morgan died en route to hospital after they lost control of their 14m (46ft) Big Thunder Marine, which is powered by four 1,200bhp engines.
Superboat International president John Carbonell said the force of the boat hitting the water was ‘powerful’ and ‘crushing’.
He said: ‘He [Tillman] was probably going about 130mph and the boat’s propellers were barely in the water.
‘The boat apparently caught some air and went bow up, straight into the air, came down and went backwards.’ The accident marred the third day of racing at the Key West World Championship in Florida.
Despite the accident and rescue efforts, the race later continued until another accident on the seventh lap.