There is "no suggestion" of people jumping from Bournemouth Pier before an incident that left two children dead.
Dorset Police's assistant chief constable Rachel Farrell read a statement following the heartbreaking incident that happened yesterday afternoon.
A 12-year-old girl from Buckinghamshire and a 17-year-old boy from Southampton were pulled from the sea but later pronounced dead.
ACC Farrell said the children are not related.
Another eight people were rescued and treated for non life-threatening injuries.
A man in his 40s has been arrested on suspicion of manslaughter.
The assistant chief constable said: "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."
Dorset Police faced questions over why so little information is being provided about the incident and ACC Farrell said this was to protect the ongoing investigation.
On Wednesday evening police said: "Dorset Police received a report at 4.32pm on Wednesday 31 May 2023 from the ambulance service requesting assistance on the beach off Bournemouth Pier.
"Emergency services attended and ten people were recovered from the water.
"A 17-year-old boy and a 12-year-old girl sustained critical injuries. Very sadly, they both later died in hospital. Their families have been informed.
"The other eight people were treated by the ambulance service for injuries that are not life-threatening.
"An investigation into the circumstances surrounding the incident has been launched with Dorset Police working alongside the Marine Accident Investigation Branch and the Maritime and Coastguard Agency in accordance with established protocols.
"Following enquiries, a man aged in his 40s who was on the water at the time has been arrested on suspicion of manslaughter."