Two men from the United States and one from Norway died when the shark diving boat they were in capsized near Kleinbaai on Sunday, the National Sea Rescue Institute (NSRI) said.

Spokesperson Craig Lambinon said a 36-foot White Shark Projects boat capsized off-shore of Kleinbaai near Gansbaai with an estimated 19 crew on-board.

"Seventeen people had been rescued out of the water by local private boats and five of the rescued had been taken for treatment.

"One was declared dead on the scene after extensive efforts by paramedics to resuscitate him were terminated." he said.

Local boats brought in the remaining two male passengers who were both declared dead on arrival by paramedics, said Lambinon.

Chairwoman of the Great White Shark Protection Foundation, Mariette Hoply, said the shark-cage diving boat had anchored just before a freak wave capsized it around 10am on Sunday.

The boat's crew had not started shark-cage diving when the wave struck. Ten people on board were clients of Great White Shark Projects. The other nine people were crew and staff members.

One of the people swept off the boat was caught underneath the boat and had to be retrieved by divers.

Sapa