A 12-year-old girl and a 17-year-old boy who died after being pulled from the sea off Bournemouth beach had "no physical contact" with a vessel on the water, police said.
Dorset Police said it was still "investigating the circumstances that caused a number of swimmers to get into difficulty" on Wednesday.
Eight other people were treated for non-life-threatening injuries.
A man in his 40s has been arrested on suspicion of manslaughter.
Emergency services were called to the beach, packed with people on half-term holidays, at 16:32 BST.
Dorset Police has released more information as officers work to piece together exactly what happened.
The force said it was working with partner agencies to understand what "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," it said in a statement released on Thursday.
Det Ch Supt Neil Corrigan also asked people not to speculate about the circumstances.
The children pulled from the water sustained critical injuries and died in hospital later.
The coastguard conducted a search to make sure no other people were missing and said it was "satisfied there are not".
Police confirmed the man arrested was "on the water" at the time of the incident.
Some eyewitnesses told the BBC lifeguards began to tell people to clear the beach, saying there had been a major incident.
A section of the beach and the nearby Pier Approach were cleared and a cordon was put in place.
Emergency services including the air ambulance, police, fire service and South Western Ambulance Service were all called to the scene.
The Marine Accident Investigation Branch and the Maritime and Coastguard Agency are also involved in the investigation.
Dorset Police is appealing for witnesses to come forward and has said further information will be released "as the investigation progresses".
Tobias Ellwood, the Conservative MP for Bournemouth East, said protocols on the pier could be reviewed after the "terrible tragedy".
He said Bournemouth prided itself on being a family resort and the incident had taken "everybody by shock".
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