Fisherman dies as son survives SA capsize

The death of an elderly fisherman whose boat capsized off South Australia's coast remains a mystery, with police unsure what caused the vessel to overturn.

The 72-year-old was found dead in the Gulf St Vincent early on Monday after a police helicopter spotted the distressed boat.

His 44-year-old son was found clinging to the hull and was rescued by the volunteer Coast Guard around 1.30am.

Police launched an air and sea search after family members reported the pair overdue from their fishing trip.

It remains unclear what caused the large vessel to capsize in what were believed to be calm conditions.

"It was a 5.5 metre boat. It is reasonably heavy and appears to be a well-constructed boat," Brevet Sergeant Mark Jacobs told reporters on Monday.

"It appears as a complete tragedy for all involved."

Police said a life-jacket was found on board but would not confirm whether either man was wearing one.

A report will be prepared for the coroner.