A 12-year-old girl and a 17-year-old boy have died after being pulled from the sea off Bournemouth Pier.
Eight other people were treated for non-life-threatening injuries after the incident, to which emergency services were called at 16:32 BST on Wednesday.
A man in his 40s who was on the water at the time has been arrested on suspicion of manslaughter, Dorset Police said.
An investigation has been launched into what happened.
It is not clear how the boy and girl, who died in hospital, sustained their critical injuries.
Dorset Police are working alongside the Marine Accident Investigation Branch and the Maritime and Coastguard Agency.
The coastguard said it had conducted a search to make sure there were no other people missing and was “satisfied there are not”.
In a statement, Detective Chief Superintendent Neil Corrigan said the beach is understood to have been “very busy” at the time of the incident and appealed to anyone with any information to come forward.
“We are at the early stages of our investigation and would ask people not to speculate about the circumstances surrounding the incident,” he said.
He added that the force was doing “all we can to support” the families of the two people who died.
Other emergency services that attended the scene included South West Ambulance Service, two air ambulances, Poole and Southbourne coastguard rescue teams, and a Mudeford RNLI lifeboat.
The beach was also cleared, along with the nearby Pier Approach, with a cordon set up.
Eeman Qamar, from Southampton, was on the beach with her mother and three-month-old baby at the time. She told the BBC that, just after 16:00 BST, lifeguards began to tell people to clear the beach, saying there had been a major incident.
“After about 20 minutes, the first air ambulance arrived and landed right in the middle of the beach,” she said.
“The lifeguards started getting on jet skis and boats, searching the sea and about 20 minutes later the second air ambulance arrived and it took another hour-and-a-half for them to finish the whole search and rescue operation.”
Kathryn Walton, from Oxford, was also on the scene with her family and described seeing lifeguards and “loads of people rushing on to the beach”.
She said people were moved away from an area of the beach as several other emergency service vehicles arrived.
Another eyewitness, Trevor Pinto, had been walking along the pier with his 16-year-old son and said the incident happened “very close” by.
He said they watched as lifeguards attempted to resuscitate two people, adding: “It took me a while to realise, oh my god someone had lost their life”.
Both the emergency services and local authority seemed to be “very well organised” and “properly prepared” for such an incident, he said.
Bournemouth West MP Conor Burns expressed his condolences to the families of those who died and said the incident was a “salutary lesson that our beaches and ocean can give much pleasure but danger is ever present”.
“A dreadful event in circumstances when they were enjoying beautiful weather in our town. So sad,” he wrote on Twitter. “Thanks to the lifeguards and the Air Ambulance who we can take for granted.”
Anyone with any information is being asked to contact Dorset Police via the force’s website or by calling 101, quoting occurrence number 55230083818. Crimestoppers can also be contacted anonymously via its website or on freephone 0800 555 111.
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