Sydney electrician Raveshan Pillay, his British girlfriend Danielle Hooker, her father Nigel and 21 other tourists were watching sea lions on a reef off Canada's Vancouver Island when the master and a deckhand on the Leviathan II whale watching vessel heard a noise.
They looked to the rear of their 20m long boat and bearing down on them was a large breaking wave.
The top of the wave was higher than the vessel's flying bridge.
The horrifying details are in a report on the 2015 tragedy released on Wednesday by Canada's Transportation Safety Board.
It describes how the boat's master placed the vessel in the path of potential waves and then urgently attempted to lessen the impact when the big wave suddenly appeared.
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The report also describes the chaos that followed as Pillay and other passengers were tossed into the frigid 14 degree Celsius water and the Leviathan II rapidly capsized.
Mr Pillay, Mr Hooker and four other tourists, all British, died.
Ms Hooker survived.
Theories of a rogue wave or whale striking the Leviathan II were ruled out by investigators.
"On the afternoon of October 25, 2015, a large breaking wave struck the passenger vessel Leviathan II, causing it to broach and rapidly capsize throwing 27 passengers and crew into the cold waters off the west coast of Vancouver Island," TSB chair Kathy Fox told reporters at the release of the report in Vancouver.
With the wave bearing down on the boat, the master reached for the throttles in an attempt to turn the vessel so it would encounter the wave at its rear.
However, the wave struck the vessel's starboard quarter, causing it to broach and capsize.
Some passengers slid down the top open deck as the vessel capsized, hitting railings, seats, and other objects before entering the water.
The master and the deckhand were initially trapped inside the flying bridge before escaping.
Some passengers ingested sea water upon being submerged and some were under water for as much as one minute.
The sudden capsizing resulted in passengers and crew hitting the water without flotation aids or thermal protection and it took about 45 minutes before search-and-rescue authorities became aware of the tragedy
Breaking waves were not "seen when the vessel first approached the area to observe sea lions", although the report pointed to conditions that would support the formation of breaking waves.
The swell was coming from the south-east, travelling over the rising ocean floor and meeting an opposing tide as it approached the rocks.
"Based on the conditions at the time, the master deemed it safe to operate the vessel on the weather side of the reef, and during these operations, the vessel took on a position and heading that exposed the vessel's starboard quarter to the incoming waves," the report states.
At the time and location of the occurrence, the wind was estimated to be 10 knots from the southeast, and wave conditions were a 2m swell from the same direction.
