Blake McKenzie Sinclair (18) drowned in the Clutha River near Balclutha after he was dislodged from his knee-board and ''face planted'' into the water on January 12.
In his formal written findings released yesterday, Otago-Southland coroner David Crerar said Blake and his family decided to go knee-boarding on the river in the afternoon of January 12.
The Waitahuna River, where they launched their jet-boat, was described as calm because it was backed up against the flooded Clutha River.
Blake, his brother Logan, and father Mark took turns driving the boat up and down several rivers. The river conditions were considered and the brothers decided to go knee-boarding, each taking off their lifejackets and putting on wetsuits.
Mr Crerar said although wetsuits provided some flotation, it was unusual for the Sinclairs not to have worn lifejackets as well.
Logan went knee-boarding first, before swapping places with Blake.
As Blake got into the water, he was reminded by his mother to put on his lifejacket, but he did not.
About 100m downstream from the Waitahuna confluence - the point where the Waitahuna River meets the Clutha - Blake skipped over the wake of the jet-boat, becoming airborne ''about a foot off the water''. He was lost control, let go of the rope, and ''face planted'' into the water, Mr Crerar said.
He did not surface.
Mark Sinclair immediately dived into the water, but could not see or feel anything as the river was dirty and in flood.
The family called emergency services and kept searching. A search and rescue operation was launched, and continued for seven days.
Blake's body was not recovered until the following Saturday, caught in willow branches on the true left of the Clutha River at Manuka Island.
Mr Crerar found Blake's cause of death was drowning, but it was possible he was stunned and unable to get back to the surface of the water.
In a letter to Mr Crerar, Maritime New Zealand said Blake's death was not likely to result in any new safety messages about recreational boating, but Mr Crerar said it was important to repeat those messages.
''If he had of [sic] been wearing a lifejacket, it is probable that he would have immediately been found and recovered as soon as he fell from the knee-board.''
He said he hoped the tragedy would draw attention to the dangers involved with boating without wearing an appropriate personal flotation device, or lifejacket.
