Three tourists drowned on Sunday after a shark diving boat was hit by a "freak wave" and capsized half a nautical mile off Kleinbaai, near Gansbaai.
Two were seriously injured and a number treated for shock.
There were 19 people on the 11-metre catamaran Shark Team - 10 foreign tourists and nine crew members. Two of the victims were American, and one Norwegian. The names were not released pending family notification.
"Our hearts and sincere feelings go out to the families. It's obviously been very traumatic for all involved," said Charmaine Beukes, owner of White Shark Projects, which operated the boat.
The water was calm when the boat went out early in the morning. "The sea was flat, absolutely flat. It was fantastic conditions to go out to sea, and this wave actually came out of nowhere," said industry spokesperson Mariette Hopley, chairperson of the Great White Shark Foundation.
There was a southeasterly wind of 10 to 15 knots and an approximately two-metre swell, according to the NSRI.
A British survivor, who asked not to be named, said the boat was preparing to return to shore about 10.15am when he and his wife saw a huge, "tsunami-like" wave 100 metres away. It was his first time shark-diving, "something I had always wanted to do".
He and his wife watched another shark diving boat ride over it, he said, but as it came near them the captain realised that they were in trouble.
He said that the skipper shouted to them to grab hold of something, and then the wave crashed over the boat, causing it to roll.
By the time he and his wife broke the surface another shark diving boat was already there rescuing people. His wife fractured a shoulder in the accident.
"I've never seen anything like it. Nothing in the world would have stood a chance, except maybe an ocean liner. You don't even have time to think," he said.
One of the victims was trapped underneath the boat, Hopley said. He was found by divers and taken to shore. Paramedics found a weak pulse and tried to resuscitate the man in an ambulance so that he could be stabilised and airlifted to hospital, but they were not able to and he was pronounced dead in the ambulance.
The other two men who drowned were found in the water, taken to shore by a rescue boat, and pronounced dead on arrival.
Two passengers were taken to hospital, one with a broken foot and the other with a shoulder injury, while others were treated for shock.
Hopley said that all eight local shark diving companies operate in the same designated area, and that several other boats were nearby when the wave hit.
Videographer Sarah Dix was aboard the White Pointer, a boat operated by Unreal Dives, which had just arrived and was preparing to drop anchor when the wave passed through: "It was massive. This wave was just huge."
Dix said she saw the large swell pass by another shark-diving boat. She was still looking at that boat to ensure that its crew and passengers were safe when she noticed the Shark Team had overturned 10m away.
Dix said the Shark Team's skipper was gathering passengers, getting them on to the overturned hull. Other passengers were pulled onto another shark diving boat: "The crew were just hauling them on. There were actually sharks around, so it was a case of getting as many people out of the water."
Hopley said that none of the bodies had any injuries or physical marks that would suggest that they had been attacked by sharks.
Dix said the accident happened quickly, and the rescue effort was instant: "It was so under control... It was dealt with very well."
The passengers were not wearing lifejackets, Beukes said, and none of the companies in their area require passengers to wear them at all times.