The man’s wife and three other children are being treated in hospital for their injuries, some of which were described by Devon and Cornwall Police as “life-changing”. The family, from London, was in Cornwall for the Bank Holiday weekend when the accident happened in the Camel Estuary near Padstow.
Emergency services said the man and girl were declared dead at the scene. A 39-year-old woman, a four-year-old boy and two girls aged ten and 12 were airlifted to Derriford Hospital in Plymouth with a range of injuries.
Senior investigating officer Superintendent Jim Colwell, of Devon and Cornwall Police, said some of the injuries were “life threatening” as well as “life- changing”.
The force has launched an investigation and the boat, which was owned by the family, has been taken away for examination.
Dozens of members of the public are believed to have witnessed the accident, in which the victims were hit by their “out of control” boat.
Maritime and Coastguard Agency spokeswoman Jo Rawlings said: “There were reports that six people had been thrown off a speedboat, and that the speedboat was out of control for a short time.”
999 operators received their first call from a mobile phone at 3:48pm and were inundated with more calls seconds later. At about that time, the popular harbour area was bristling with people enjoying the sunshine, who were able to see the incident just half a mile off the shore.
Matt Pavitt, the Coastguard sector manager for North Cornwall, said the injured four were “badly shaken up”. He added: “There appears to have been some interaction between the boat and the group of people in the water, which has resulted in a number of serious injuries.”
Lifeboats from Rock and Padstow, coastguard rescue teams, and search and rescue helicopters were all called to help the group.
Last night witnesses claimed heroic actions by passers-by prevented more lives being lost.
Writing on Facebook, Maria Chase said: “We were down in Padstow today and saw the tragedy and saw a hero who saved more lives by jumping on to an out-of-control speedboat from another boat.”
Other witnesses described the terrible accident as like being a scene from a horror film.
Local resident Martin Jenkins, 63, said: “I heard the helicopter and went over to the estuary and saw the ambulance crews working on a couple of people. It was all very frantic and the people were lying on the floor in a lot of discomfort.
“They were airlifted away, I think, because they were put on some stretchers. It looked terrible, the water was filled with blood, like in a horror film.”
One woman, who did not want to be named said: “We rushed over to see the commotion and saw two ambulances on the beach, they were working on two people who looked in a lot of trouble. The water was red – it was terrible. That boat most have hit those poor people at a terrifying speed.”