A 51-year-old man and his daughter, eight, have died after a speedboat crashed off the north Cornish coast.
Four other family members were seriously injured when they were thrown from the boat off Padstow harbour. They have been airlifted to hospital.
Children and adults suffered leg injuries of varying degrees of severity, John Oliver from South Western Ambulance Service said.
Police are investigating whether the boat hit kayakers after losing control.
The four injured are a 39-year-old woman, a four-year-old boy and two girls aged 10 and 12.
The boat is owned by the family, who are from London, police said.
Supt Jim Colwell of Devon and Cornwall Police said some of their injuries were "life threatening" as well as "life changing".
He praised the agencies involved in the rescue operation.
"I have to thank them for the work that was done, in very difficult and distressing circumstances. The agencies responded really effectively.
"This is a tragic incident, but it could have been worse."
The sunny bank holiday weather had drawn a lot of visitors to the harbourside, and the surrounding waters of the Camel Estuary were said to have been calm on Sunday afternoon when the accident happened.
At about 15:50 BST, Falmouth coastguard received a number of reports from members of the public that six people had been thrown from a speedboat.
They reported seeing the boat "out of control for a short time" and striking some other boats, Jo Rawlings, from the Maritime and Coastguard Agency, told the BBC.
Dog walker Simon Lewins, from Wadebridge, said he watched as a big, powerful boat going "a bit too fast" suddenly turned right, "depositing" people into the water.
"It kept going off in ever decreasing circles. The screams coming from the people in the water were pretty bad."
He said one of the coastguards then jumped in and managed to stop the boat and take it away.
"I tell you what, this guy is a hero!" he told the BBC.
Coastguards then helped some of the injured as a helicopter landed on the beach, he added.
The injured were being treated at Derriford Hospital in Plymouth, the Maritime and Coastguard Agency said.
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.
There will be an investigation into the circumstances leading up to the crash, and what caused the occupants to enter the water, police said.