Feeling people to his left and right, who he believed were Mr Stoneham and Mr North, he "grabbed one of them and started shaking them and there was no response and that freaked me out".
In the darkness Mr Henry felt someone grab at his regulator and then let go, causing him to swallow water.
"Now I was thinking I am going to die. I was fully panicking and scared."
Untangling himself from the rope, he managed to surface to find everyone except Mr Stoneham and Mr North accounted for.
The possibility the men were trying to win a bet by being first to the bottom was raised at the inquest.
Mr Henry said he was not aware of a bet but there was some discussion between the pair before the dive: "I think they were saying 'meet you at the bottom' or something."
"We were just trying to get the dive over and done with," he said.
The dive course was run by Helix Training.
An investigation after the deaths resulted in a number of changes, but none would have altered the tragic outcome of the exercise, Helix Training director Haydon Allan told the inquest.
He considered the men who led the eight-member dive party - instructor Teauriki Tuaana and assistants Rawiri Anderson and Tangata Vaikai - were experienced, and he had no concerns about the abilities of anyone taking part.
Mr Allan was overseas at the time of the incident, but said the investigation revealed several factors contributed to the tragedy including the divers not following the brief or buddy procedure.
Divers were always paired with a buddy for safety and were briefed on guidelines to follow if they lost their fellow diver.
In addition, the instructor did not carry a knife, no lights were issued and not all students were issued dive computers, all of which were outside diving policy, he said.
Knives are used for freeing the diver or their buddy in an emergency and lights help divers when visibility is poor.
A dive computer measures the time and depth of a dive to help calculate a safe ascent.
The company's students were now issued with cutting items, carried lights and were issued with diving computers during deep dives.
The inquest was told conditions on the day of the dive were poor, with visibility under the water extremely limited.
In his several dives in Lake Pupuke, Mr Allan said he had never experienced blackout conditions, other than when they were man-made by stirring up the bottom of the lake.
Rawiri Anderson, a student and dive control specialist, broke down as he spoke at the inquest and said the dark conditions were mentioned just once.
He said Tangata Vaikai checked what it was like beneath the surface before the dive began and warned the divers to stick together.
Mr Anderson told the inquest no one thought torches were needed for the exercise as it was a day-time dive and past dives at the lake had not required extra lighting.
When asked by Coroner Gordon Matenga why the dive went wrong, he said it was probably due to "no ears", meaning people weren't paying proper attention.
Mr Vaikai told the hearing that once the dive began, Mr Vaikai said the usual circle formation undertaken by diving groups fell apart.
"It was getting dark, and getting dark really fast.
"And everyone was dropping really fast."
The inquest continues.
