ITV News Meridian's Richard Slee reports live from Bournemouth where two children have died.
Police say there is no suggestion of people jumping from the pier at Bournemouth beach following the deaths of a 17-year-old boy and a 12-year-old girl.
Speaking to the media on Thursday, Dorset Police Assistant Chief Constable Rachel Farrell confirmed they are not investigating reports of jetskis being involved.
Police have said that the two youngsters who were pulled from the water were not related, and were not known to each other. The children were from different families.
Assistant Chief Constable Rachel Farrell thanked emergency services and members of the public who helped, and say they are working alongside the Marine Accident Investigation Branch with their enquiries.
Dorset Police Assistant Chief Constable Rachel Farrell gives an update following the deaths of a 17-year-old boy and a 12-year-old girl
Assistant Chief Constable Farrell also confirmed that there was no physical contact between swimmers and a boat.
A 17-year-old boy from Southampton and a 12-year-old girl from Buckinghamshire were taken to hospital following the incident on Wednesday afternoon, but they later died from their injuries.
Eight others were treated at the scene, but none of them were taken to hospital.
Multiple emergency services including police, two air ambulances and the RNLI were called to the scene.
A man aged in his 40s, who was on the water at the time, has been arrested on suspicion of manslaughter.
Assistant Chief Constable Rachel Farrell said: "Following our initial inquiries, a man aged in his 40s, who was on the water at the time, has been arrested on suspicion of manslaughter.
"As we have a person in custody, you will know there is only limited information I can give about the investigation.
"However, to avoid further speculation, I am able to tell you that it's clear that yesterday a number of people already in the water got into difficulty and we are investigating the circumstances or event that caused that to happen.
"Early investigation indicates that there was no physical contact between a vessel and any of the swimmers at the time of the incident.
"I can also confirm there is no suggestion of people jumping from the pier or jet-skis being involved.
"We are all truly devastated that two young people have lost their lives.
"My thoughts and those of all the emergency services are very much with their loved ones at this horrendous time and we are doing all we can to support their families.
"As you can imagine, we are at the early stages of our investigation, and we would ask people not to speculate about the circumstances surrounding the incident to both protect our inquiries and out of respect for the victims and their families.
"We are aware of a number of videos circulating on social media and we would urge people to refrain from doing this. We know the beach was very busy when the incident occurred. I am today urging anyone who saw what happened or has any information that may assist to please come forward.
"If anyone has relevant phone footage then they can contact Dorset Police through our major incident public portal where they can upload the images and we will share the links on social media.
"This operation is named Operation Marble. So please share the images with the police rather than on social media.
"I would sincerely like to thank members of the public who helped people in trouble in the water and also I am very grateful to wider beachgoers who really quickly moved from the beach to allow emergency workers do their work."
A spokesperson for the RNLI said: "We would like to offer our condolences to the family and loved ones of the two young people who sadly lost their lives yesterday at Bournemouth beach.
"RNLI lifeguards along with volunteer lifeboat crews from RNLI Mudeford worked with emergency services to provide search and rescue, and casualty care.
"This incident is now in the hands of Dorset Police and we will not offer any further comment at this point."
Dozens of people lined the promenade after the beach was evacuated.
An eyewitness reported seeing a group of teenagers jumping from the pier into the water. He said it appeared that they were being pulled under the pier by the strong current.
He added that two people were seen being helped to shore by members of the public, and treated by medical crews.
Police are urging people not to speculate on the circumstances surrounding the incident.
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 enquiries 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.”
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know...