The two youngsters who died off Bournemouth beach had not jumped from the pier or been hit by a jet-ski, police have confirmed.

A 17-year-old boy from Southampton and a 12-year-old girl from Buckinghamshire died after getting into difficulty in the water at the Dorset resort on Wednesday afternoon.

A further eight people were treated by paramedics following the incident.

A man in his 40s, who was “on the water” at the time, has been arrested on suspicion of manslaughter.

A sightseeing boat, the Dorset Belle, has been impounded at Poole Harbour by police, according to reports.

A police source told The Times: “This vessel is under a police cordon. No-one is permitted onboard or to touch the vessel.”

Speaking at a press conference, Dorset Police Assistant Chief Constable Rachel Farrell confirmed that none of the swimmers had been in “physical contact” with a vessel and they had not jumped from the pier.

She added that speculation that jet-skis had been involved was also incorrect.

“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,” she said.

“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.”

She said the two who died were not related to each other and those involved were “different people from different families” who were assisted by “great-spirited members of the public”.

She added: “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.”

Ms Farrell added: “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.”

She added: “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.”

Bournemouth, Christchurch and Poole Council leader Vikki Slade said the authority is working with the emergency services and will be assessing safety in the water near the pier.

“There are specific rules relating to the buoys in the water but there is no evidence that any of these rules have been breached,” she said.

Families arriving at the beach on Thursday spoke 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, it’s a real tragedy.”

Mourners were seen placing a bunch of flowers on the beach close to the pier early on Thursday morning.

Eyewitness Howard Cohen, from London, said: “I could see a lot of commotion going on; there were a lot of ambulances, police and people rushing to the beach.

“Later on they cleared the beach. It was just awful. At the time there was a lot of panic going on with the police and ambulance.”

Another 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 jet-skis 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.”

Bournemouth West MP Conor Burns said the incident is a “salutary lesson” that “danger is ever present” on beaches and the ocean.