A Blackpool man died after being found unconscious by the Coastguard.
The 46-year-old diver was treated at the scene, Weymouth Harbour in Dorset, at around 11.20am yesterday, but paramedics were unable to save him.
He was pronounced dead "a short time later", the local force said in a statement.
"His next of kin have now been informed and the coroner notified," it added.
"The death is not being treated as suspicious and enquiries are being carried out on behalf of the coroner."
The man's identity has not been released.
The BBC said the man was rescued seven miles off Portland, citing emergency services and the Coastguard.
Both the Coastguard helicopter and the air ambulance were called out, the Dorset Echo reported.
The diver was understood to have been brought into the harbour on the dive boat with a paramedic on board, it added.