Kevin Reynolds, 52, and his 13-year-old daughter lost the two Jack Russells in the water off Brighton seafront just after midday on Saturday.
A Maritime and Coastguard Agency spokesman said Mr Reynolds went in first to try to help the animals when they were spotted. But it appears that he fell and could have hit his head on a groyne.
The teenager, along with several members of the public who witnessed the drama, also then went into the water to help her father.
She was quickly rescued in the rough seas but Mr Reynolds, who lived in Brighton, was washed out more than 30 metres towards the pier in sea temperatures of 5C.
He was lifted from the sea by the Brighton RNLI inshore lifeboat and then airlifted to the Royal Sussex Hospital, where medics were unable to save him after three hours of treatment.
His distressed daughter was treated by ambulance crews but she was not seriously injured. One of the dogs has been found. The other is missing.