He dived under the water to have a look and saw Mr de Vries attempting to pull in the kingfish, but he did not appear to be in any trouble, Mr Smith said.
Mr Masson then had to look away because he had become tangled in a line - when he looked back to his partner, Mr de Vries had disappeared.
The second boat of friends, including David Mullens, who was a competitive and experienced free diver, helped in the search.
Mr Mullens eventually found Mr de Vries in 27m of water and returned his body to the surface, Mr Smith said.
A police dive squad report to the coroner described a "shallow water blackout".
"This is where there is a loss of consciousness caused by a reduction of oxygen to the brain.
"Shallow water blackout occurs in spear fishing when reef hold divers descend for a period of time. During the return ascent, the changes in pressure and reduced oxygen content starve the diver's brain and cause them to blackout."
"The [police] report considered that Mr de Vries like all experienced divers, understood and accepted the risks associated with this sport," Mr Smith said.
Mr Masson could not have foreseen what would happen the moment he decided to free himself from the rope that had entangled him, he said.
It was a tragic death that happened even when a spear fishing operation was being carried out in a "safe and professional manner", Mr Smith said.
"It simply always highlights that there will be a danger when carrying out water sport activities."
He recommended that when using the "one up, one down" diving principle, it was important to ensure the diver who was monitoring from the surface maintained visual contact with the submerged diver until they surfaced and remained surfaced for at least a minute.
The diver should also manage their weight so they could maintain a "neutral hover point" in the water at about 30 to 40 per cent of the death they would dive, Mr Smith said.
