A 51-year-old man and his eight-year-old daughter were killed and four other members of their family airlifted to hospital after a speedboat accident, Devon and Cornwall Police said tonight.
A major search and rescue operation got under way after the incident in Padstow harbour, north Cornwall.
South Western Ambulance Service said two people had died and four were seriously hurt.
A spokeswoman for the Maritime and Coastguard Agency said: "Falmouth Coastguard was first called at 3.50pm when there were reports that six people had been thrown off a speedboat, and that the speedboat was out of control for a short time."
A Devon and Cornwall Police spokesman said officers were called to the scene in the Camel Estuary, near Padstow.
He said: "It's still an ongoing incident and we are working alongside the coastguard and ambulance service."
South Western Ambulance Service said they were called to reports that a speedboat had come into contact with a number of kayakers.
Ambulance service spokesman John Oliver said those injured were children and adults with leg injuries of various degrees of severity.
It is not yet known if they were in the speedboat or in the water at the time of the collision.
The coastguard spokeswoman said initial reports were that the boat had struck a number of people in the water and it was thought these might have been thrown off the boat.
The search and rescue operation was undertaken by RNLI lifeboats from Rock and Padstow, Cornwall, and helicopters from RNAS Culdrose, Cornwall and RMB Chivenor, Devon, plus three coastguard rescue teams from Polzeath, St Merryn and Newquay, all Cornwall.