A man who was arrested on suspicion of manslaughter over the deaths of a teenager and a 12-year-old girl has been released under investigation while enquiries continue, police said.
A 17-year-old boy from Southampton and a girl from Buckinghamshire died following an incident on Wednesday afternoon. They were not related, officers have confirmed.
The man arrested was on the water at the time, Dorset Police said.
Eight other people were treated at the scene for non-life threatening injuries.
Officers dismissed suggestions a boat hit swimmers during the major incident off Bournemouth beach.
The force said today that "early investigation indicates that there was no physical contact between a vessel and any swimmers at the time of the incident".
Detective Chief Superintendent Neil Corrigan said: "Our thoughts remain very much with the loved ones of the two young people at this horrendous time and we are doing all we can to support their families.
"We are continuing with our investigation, which is still at the early stages, and would ask people not to speculate about the circumstances surrounding the incident to both protect our enquiries and out of respect for the victims' families.
Police were seen standing by a pleasure boat called the Dorset Belle, moored by Poole Harbour, on Thursday evening.
According to the online tracker, Marine Traffic, on the day of the deaths the boat made numerous circuits in the waters off Bournemouth Pier, including around the time of the incident.
At a press conference in Bournemouth, Assistant Chief Constable Rachel Farrell also said there was no suggestion that people were jumping into the sea from the pier or that jet skis were involved.
"We are working with partner agencies to understand what has caused the tragedy," Dorset Police said in a statement.
"We are investigating the circumstances that caused a number of swimmers to get into difficulty. Further information will be released as the investigation progresses."
Tom Saunders, a surfer who helped with the search operation, said he heard "lifeguard sirens going off, making an announcement, and then the beach got cleared either side".
"So obviously, I knew something had gone on. We heard the helicopters coming so knew it was serious," he said.
Mr Saunders and three other surfers searched areas of the pier where the lifeguards' jet skis could not reach.
"They said there were two people missing at the time and then after about a half hour they said everyone was accounted for," he added.