A 40-year-old man has been arrested after a body was found floating in the water near Bournemouth pier.
Speedboats are restricted to certain areas of the beach
The body of the 27-year-old single man, who lived and worked in London, was found 300 metres from the pier, in an area restricted to swimmers.
It is believed he was struck by a vessel possibly, a speedboat at 1800 BST on Sunday.
Dorset Police arrested the man from Poole on suspicion of manslaughter at 1330 BST.
Officers are currently examining a boat in connection with the incident.
Sergeant Andy Hack, from the Dorset Police Marine Section, said the man was recovered from the sea by lifeguards and taken to Poole Hospital where he was pronounced dead at 1955 BST.
The dead man has not yet been named and a post-mortem examination is being carried out on Monday.
Detectives are appealing for members of the public who may have witnesses the incident to contact them immediately.
They particularly, want to speak to people who may have videoed or taken pictures of the sea to the east of the pier, from the pier, the beach or the cliff top at about 1800 BST.