The driver of a boat which ran over and killed an 11-year-old girl he had been towing was not aware she had fallen into the water, a marine accident report said.
He did not see her as he continued on a tight circular route, the report from the Marine Accident Investigation Branch (MAIB) said.
The ski boat ran over Mari-Simon Cronje and its propeller caused severe injuries in the accident at Princes Club Water Sports Park in Bedfont, west London.
Mari-Simon, who had fallen from a "banana boat" - an inflatable yellow craft towed behind a speed boat - was pronounced dead soon after arriving at hospital.
"The implementation and execution of the safety management system used at Princes Club was flawed at every level and had not identified or controlled the risks to children taking part in banana boat rides effectively," said the MAIB report into the accident, which happened on September 11 last year.
The report added that the ski boat was operating without an observer and the driver was dividing his attention between looking ahead at where he was going, and behind to check on the welfare of those riding the banana boat.
It said the helmet issued to Mari-Simon was a grey colour that was difficult to see in the lake water.
The report also said the tight circuit taken by the driver and the limited forward visibility from the boat at the slower speeds required for towing an inflatable, reduced the opportunity to see the girl in the water.
The MAIB also said there was no oversight of operators that provide towed inflatable rides on a commercial basis and no assurance that their operating standards control the risks effectively.
Also, the report said the the licensing requirements for ski boat drivers and ski boats operating on a commercial basis were unclear.