A 17-year-old boy and a girl aged 12 have died following an incident off Bournemouth beach, Dorset Police said.
A man in his 40s has since been arrested on suspicion of manslaughter. Police confirmed that the man was "on the water" at the time of the incident.
The boy and girl had "no physical contact" with a vessel on the water, police say.
The force said it was still "investigating the circumstances that caused a number of swimmers to get into difficulty".
Paramedics sent a report to police at around 4.30pm on Wednesday afternoon of people requiring assistance on the beach off Bournemouth Pier.
Emergency services attended the scene and 10 people were recovered from the water.
The force said that a 17-year-old boy and a 12-year-old girl sustained “critical injuries”, and later died in hospital. The other eight people were treated for injuries that are not life-threatening.
A force spokesman said on Thursday: “We continue to investigate the circumstances surrounding the tragic incident that occurred yesterday resulting in the deaths of a 12-year-old girl and 17-year-old boy.
“We are working with partner agencies to understand what has caused the tragedy.
“However, early investigation indicates that there was no physical contact between a vessel and any swimmers at the time of the incident.
“We are investigating the circumstances that caused a number of swimmers to get into difficulty.
“Further information will be released as the investigation progresses.”
Families who have arrived at the beach on Thursday have spoken of their shock at the previous day’s events.
One woman, who did not want to be named, said: “It’s such a shame, people just come here to have fun during the half term, it’s a real tragedy.”
Detective Chief Superintendent Neil Corrigan, of Dorset Police, said: “Our thoughts are with the loved ones of the young people who tragically died and we are doing all we can to support their families.
“I understand the beach was very busy at the time of the incident and I would ask anyone with information that may assist our inquiries to please come forward.
“We are at the early stages of our investigation and would ask people not to speculate about the circumstances surrounding the incident.”
Bournemouth West MP Conor Burns said the incident was a “salutary lesson” that “danger is ever present” on beaches and the ocean.
“Thoughts with everyone associated with the dreadful events unfolding in Bournemouth this evening,” he said.
"A salutary lesson that our beaches and ocean can give much pleasure but danger is ever present.
“Thanks to the lifeguards and the Air Ambulance who we can take for granted.”
Witness Tom Saunders told Sky News: “I was surfing on the west side with about four other people and we heard the lifeguard sirens going off, making announcements and the beach got cleared either side.
“Obviously we knew something had gone on and the helicopter was coming so we knew it was serious.
“Unfortunately I saw someone receiving CPR, the lifeguards put up a screen so no-one could see but unfortunately from where we were we could.
“The jetskis were going round the pier looking for stuff so me and three other surfers offered to help and actually paddled into the pier to have a look.
“They said there were two people missing at the time then after about half an hour they said everyone was accounted for and we just carried on.”
