A large, steep breaking wave led to the capsizing of a whale-watching boat in Canada, and ultimately killing one Australian and five British sightseers, according to the Transportation Safety Board of Canada's report on the tragedy.
Sydney electrician Raveshan Pillay, 27, was holidaying in Canada with his British girlfriend Danielle Hooker and her father Nigel on October 25, 2015 when they boarded the 20m long Leviathan II with 24 other passengers and crew for the fatal tour.
Mr Pillay and Mr Hooker were among the dead, while Ms Hooker survived.
Luck likely saved more sightseers from drowning with crew members in the sinking vessel "by chance" retrieving and activating a flare.
The agency described at a press conference on Wednesday how the vessel had been positioned in the Plover Reefs area near Tofino, British Columbia, for sightseers to view sea lions on rocks.
The investigation determined that while sea conditions in the area were favourable to the formation of breaking waves, "none had been seen when the vessel first approached the area to observe sea lions".
"Moments after the master became aware of the large wave approaching the starboard quarter, he tried to turn the vessel to minimise the impact, but the wave struck the vessel before these actions could be effective," the TSB said in a statement.
"The crew did not have time to transmit a distress call before the capsizing, nor did the vessel have a means to automatically send a distress call.
"It was only by chance that the crew retrieved and activated a parachute flare, alerting nearby Ahousaht First Nation fishermen who arrived on the scene first, alerted search-and-rescue (SAR) authorities, and began recovering survivors from the water."
Survivors had questioned whether the vessel sank after passengers on the top deck all stood on one side to view the sea lions, exacerbating its instability when the wave hit.
However, investigators ruled that out.
Investigators also dismissed a "rogue wave" theory.
Kathy Fox, chair of the TSB, said three recommendations to improve passenger safety would be issued.
"It's time for Transport Canada to work with whale-watching companies and other passenger vessel operators to ensure the experience they offer is not just thrilling, but as safe as it can be," Ms Fox said.
"When people find themselves in cold water, every second counts.
"Our recommendations today are aimed at putting in place measures to avoid accidents in the first place, and to expedite rescue efforts if an accident occurs."
The board has recommended Transport Canada require commercial passenger vessel operators on the west coast of Vancouver Island to identify areas and conditions conducive to the formation of hazardous waves, and adopt practical strategies to reduce the likelihood of an encounter.
The board is also recommending passenger vessel operators across Canada adopt "explicit risk-management processes" to identify hazards and implement strategies to reduce these risks.
The final recommendation is aimed at reducing response time in the event of an accident.
It took 45 minutes after the capsizing before authorities became aware of the capsizing and the TSB wants all commercial passenger vessels operating beyond sheltered waters to carry emergency position-indicating radio beacons or other similar equipment.
Mr Pillay's body was found almost a month after the tragedy.