While it was impossible to judge how the effects of cannabis had contributed to, or resulted in, their deaths, it was "unlikely that the consumption of cannabis enhanced the survival chances", he said.
"Both Shaun Bethune and Lindsay Cullen ended up in the water and whilst swimming to safety succumbed to the cold water conditions.
"The cause of each of their deaths was determined at autopsy to be hypothermia due to cold-water immersion."
Maritime New Zealand had programmes stating "water and alcohol don't mix", but an education programme warning those in boats not to smoke cannabis should be added.
"The effects of cannabis can decrease survivability in cold water."
The coroner said the survival of the other three people on the boat Extreme 1 - owner Barry Bethune, a farm manager, of Southland, and sisters Carol Saxton, sales manager, of Wellington, and Denise Zonneveld, nurse, of Southland - were "directly attributable" to Barry Bethune's insistence lifejackets be worn.
Mr Crerar said that after launching the 7.2m aluminium catamaran from Bluff on the evening of January 3, Barry Bethune "ensured that all persons on board donned lifejackets".
Navigating by observation, assisted by a chart plotter, he altered his course to cater to the swell, tides and wind-changed water conditions, providing for the most comfortable and safest ride.
"When Extreme 1 reached a position about two kilometres to the west of White Island, a large wave struck the rear and side of [the boat], causing it to turn upside down. When Barry Bethune had accounted for all his passengers in the water, he attempted to dive underneath the upturned [boat] in an attempt to recover the EPIRB [emergency beacon], which was hanging by a lanyard from the throttle."
Despite several attempts he could not access the cabin of the boat, with the group at that stage swimming and drifting eastwards, attempting to reach land. They drifted to about 300m off White Island, but the current was too strong to attempt a landing and the plan was adjusted.
Mr Crerar said that as the group approached a bay in Ruapuke Island, Ms Zonneveld saw a boat - Easy Rider - moored in the bay.
"It was about this time that Shaun Bethune showed signs of distress. The others saw that his eyes were closed and that he was turning blue."
Ms Zonneveld confirmed he had died and shortly after she checked on Mr Cullen. "He was 'white and non-responsive'. Barry Bethune also checked on Lindsay Cullen and they agreed that he had also died."
The remaining three then swam towards the Easy Rider and called for help. Barry Bethune tried to climb on board, but couldn't. However, their yelling raised the attention of people on shore.
"Rewai Karetai, the owner of Easy Rider, rowed out to the survivors in a small dinghy and assisted them to shore."
Mr Karetai and seven others died in Foveaux Strait two months later when the Easy Rider capsized and sank.
Mr Crerar said it was "significant" to record that, of the three survivors - who spent more than three hours in Foveaux Strait - Ms Zonneveld and Ms Saxton "survived best", thanks to the clothing they were wearing.
The women were dressed similarly, with Icebreaker singlets, long-sleeved polo-neck fleeced tops, woollen shirts, reflective jackets and beanies.
"Barry Bethune was suffering badly from the effects of his immersion ... He was not clothed as well as the other surviving passengers."
