A Sky TV executive and his eight-year-old daughter died after he tried to execute a fast turn in their speedboat in the Camel Estuary near Padstow last May, investigators have said.
All six members of the London-based family were tossed into the water but Nick Milligan, 51, and daughter Emily were killed by the propeller blades as the boat circled out of control, the Marine Accident Investigation Branch (MAIB) said.
Mr Milligan’s wife Victoria, 39, had her leg amputated below the knee and four-year-old son Kit suffered serious leg wounds after also being hit by the boat.
Their other children, Amber, 12, and Olivia, 11, escaped with minor injuries largely thanks to water sports instructor Charlie Toogood who jumped on board from his own boat and cut the power.
The tragic accident, which took place on May 5, made headlines around the world and the MAIB said an emergency "kill cord” that would have stopped the out-of-control speedboat was not being worn.
The Milligan's went out in the 53mph boat near their £2 million holiday house at Trebetherick for the first time in 2013 having bought it the year before.
Mr Milligan, the managing director of Sky’s advertising sales division, Sky Media, had initially been at the controls and wearing a kill cord - a safety device designed to cut power in an emergency - round his leg.
Towards the end of the outing, his wife Victoria took over the controls but was not attached to the kill cord when Mr Milligan said she should make a turn.
Mrs Milligan was reluctant to do so as she did not believe there was sufficient space between the boat and the beach but due to encouragement from the children she began a slow wide turn.
Mr Milligan then reached across, in front of his wife, took the helm in his right hand and the throttle in his left, and then increased the engine throttle setting to full as he turned the helm to starboard.
The MAIB report said: "The boat immediately accelerated and heeled into the turn and then suddenly, and violently, rolled back to port and ejected all its occupants out over the port side and into the water.
"The boat then continued to circle under full power.
"The family were on the surface of the water, supported by their lifejackets and buoyancy aids, and the boat circled back towards them, striking several of them."
The report said Mr Milligan's actions appeared out of character, adding: "It cannot be established whether the wine he had consumed about one-and-a-half hours earlier adversely affected his judgment or fine motor skills when he reached across and took the controls, but his alcohol levels were well below the drink-drive limit for UK roads."
MAIB chief inspector Steve Clinch said a kill cord attached to the person at the wheel was an "essential item of safety equipment".
"I most strongly urge all powerboat drivers to ensure one is fitted and correctly worn at all times when under way, and to regularly check that it is functioning correctly," he said.
He called it a "tragic accident" and hoped lessons would be learned.
"Avoiding an accident by knowing your boat's and your own capabilities and limitations is essential, as is ensuring that all on board are safely and securely seated whenever manoeuvring or travelling at speed," he said.
Recommendations have been made to the Royal Yachting Association to include advice on the potential hazards of high power operations in its tuition and for the boat's manufacturer, APV Marine Limited, to reduce the design's steep angle of heel in tight turns.
The family said in a statement: "We are still coming to terms with this tragic accident which has left us without Nick, a loving husband, father, son and brother, and Emily, whose life was only just beginning.
"We sincerely hope that awareness of this accident will mean that another family does not have to go through anything similar."
An inquest into the deaths of Mr Milligan and his daughter has yet to be held by Cornwall Coroner Emma Carlyon.