The AHS Centaur ceremony took place at 12.45pm (AEST), almost seven hours after the Remora 3 submarine robot laid a plaque on the bow foredeck of the wreck, in what could be the last ever visit to the site.

The hospital ship Centaur was sunk by a Japanese submarine off the Queensland coast in 1943. With only 64 of the 332 people on board surviving, it was the highest casualty list of any merchant ship sunk in the Pacific in World War II.

The ship was lost for 67 years until shipwreck hunter David Mearns and his crew of 33 onboard the Seahorse Spirit sent a submarine robot named Remora 3 down 2,059 metres on Sunday to take the first confirmation footage of the wreck.

Marine historian Captain John Foley, during the ceremony, said that while the nation was at war, the Centaur should have had no reason to fear being attacked.

"Of the 332 men and women on board, only 10 needed to be awake ... the rest would be off duty and resting, if not asleep," said Capt Foley, the co-author of the book, Australian Hospital Ship Centaur - The Myth of Immunity.

"When the torpedo struck, its charge exploded, and then the bunkers exploded." Captain Foley said everyone sleeping in the forward hospital wards would have died almost instantly. "Those who did manage to jump clear of the shattered ship were showered by burning drops of oil," he said. "Many died of exposure to the sea, of burns, or injuries, or shark attacks."

Capt Foley said that whatever went through the Japanese submarine commander's mind at the time, there could be no doubt he knew what he was doing. "Centaur was brightly lit, her white hull emblazoned with a broad green banner and several red crosses that proclaimed her as a vessel of mercy," he said. "This was a measured attack. It took some time to set up the approach, to line up in readiness and to launch the torpedo."

After a prayer and a moment's silence the memorial wreath was then laid in the ocean by Army Major Arthur Dugdale. Weights on the wreath allowed it to sink to the depths of the wreck.

Yesterday , a lone felt slouch hat was recovered on the ocean floor, a very human reminder of those entombed in the Centaur. The striking image, captured with a remote-control submersible camera, is shedding light on Queensland's worst maritime disaster.

The plaque, which was donated by the 300-member Centaur Association, contains a CD with the names of everyone who served on the Centaur and personal notes from family members.

A submersible camera-robot carried the plaque to the Centaur.

"This is a gravesite and this is the marker for that gravesite," search director David L. Mearns said.

CSIRO oceanographer Dr David Griffin, who has been assisting Mr Mearns and his crew, was not surprised the hat had not been disturbed in more than 66 years.

"The current at that depth is 0.2m per second and you need a lot more than that to move something like a hat," Dr Griffin said.

Dr Griffin said the temperature at 2000m below the surface was about 4C and to go there would be like "setting a man on the moon".

"It's very cold down there, so that slows down all biological processes – it's living proof that wool can last for 60 years at depth," he said.

"Less is known 2000m below the surface than is known 20 million km away on the moon.

"Just think about how close this is to the centre of Brisbane – it's a stone's throw – but it is like setting a man on the moon to go down there."

MacArthur Museum Brisbane assistant executive officer Ron Rees yesterday identified the image as a typical army slouch hat.

He said medical officers, as well as 192 soldiers from the 2/12th Australian Field Ambulance aboard the hospital ship, would have worn the slouch hat.

Mr Rees said that with the owner of the rabbit felt hat likely to remain a mystery, it was like an unofficial memorial to all those who died.

"It's a very emotional photograph," he said. "The first thing that struck me was (that) it's like the the tomb of the unknown soldier. It's one of the most amazing photographs I have ever seen."