A speedboat had come into contact with a number of kayakers in Padstow in Cornwall, said South Western Ambulance Service.
Two people were dead when the ambulance service arrived and another four people were airlifted to Derriford Hospital in Plymouth.
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.
A father of four who was killed in a speedboat accident along with his eight-year-old daughter doted on his family", his brother has said.
Two people have died and another four people have been injured in a speedboat accident while on holiday in Cornwall.