At first, the stricken sailors were unaware of the sharks who had already started devouring their dead comrades and were now encircling them.
It had been shortly after midnight when their ship, the USS Indianapolis, was torpedoed and sunk by a Japanese submarine in the deepest part of the Pacific Ocean, some 500 miles east of the Philippines.
Of the 1,200 men aboard, about a quarter went down with her. The rest had made it into the water and in the dark hours afterwards they had struggled to keep afloat, those who didn't have lifejackets clinging to those who did, reports Daily Mail.
As their body temperatures became dangerously low, they prayed for the warmth of the morning sun.
But the dawn of July 30, 1945, brought no comfort, the daylight revealing the new terror they now faced.
"There soon were hundreds of fins around us," recalled Harold Eck, an 18-year-old seaman at the time. "The first attack I saw was on a sailor who had drifted away from the group. I heard yelling and screaming and saw him thrashing . . . then I just saw red, foamy water."
Another survivor said: "They were upon us every three or four hours." Bugler First Class Donald Mack would never forget those screams and the realisation: "There was one less man to be rescued."
The feeding frenzy which ensued remains the worst shark attack in recorded history and it is the subject of a new film starring Nicolas Cage as Captain Charles McVay, the ill-fated commander of the Indy, as it was known to its crew.
The film - USS Indianapolis: Men Of Courage - received terrible reviews in the States and has just come out on DVD here.
Shoddily made and badly scripted, it is a great disservice to the gripping testimony of the survivors, 23 of whom are still alive and who meet annually to reflect on their role in the greatest sea disaster in American naval history.
For nearly five days, they remained in shark-infested waters with no one else even realising that they were missing. This oversight was due to both human error and the nature of their mission, so top-secret that they somehow appeared to have disappeared from the U.S. Navy's radar altogether.
It wasn't until after the war that the crew of the Indy learned the hidden story of their voyage. At the time, all they knew was that they were transporting a large wooden crate from a naval yard in San Francisco to the island of Tinian, the busiest U.S. air-base in the Pacific.
The men joked that it probably contained nothing more than a consignment of luxury toilet paper for the Navy top brass.
In truth, the crate was merely a decoy for their real cargo - components for the atomic bombs about to be dropped on Hiroshima and Nagasaki. Once the delivery was done, the Indy set off for the Philippines to rendezvous with the USS Idaho for two weeks of exercises.
Although she had no sonar gear to detect submarines, Captain McVay's request for a destroyer escort was refused as the naval authorities insisted that no ships were available and that there was little risk of an attack in the waters ahead.
In fact, she was sailing straight into the path of the enemy sub which on that terrible night ripped her in two with a volley of torpedoes.
"A massive fireball flared all round me," recalled Edgar Harrell, a U.S., Marine corporal who described his experiences in his 2014 book, Out Of The Depths.
"I looked towards the front of the ship and to my astonishment it was almost gone. Men were coming up from below deck, crying hysterically for help. Many had scorched skin hanging from their faces and arms and the smell of burning flesh and hair was nauseating.
"I'll never forget the fires, the horrified faces and the cacophony of screams. I can still hear the explosions and the screeching metal being twisted and torn by the tons of water the ship was taking on."
With all electrical power cut off, no word could be sent to the engine-room to stop the Indy's vast propellers turning, so she surged on, the bows filling with water and sinking a ship nearly twice the length of a football pitch within just 12 minutes.
Some sailors were as young as 17 and when the order to abandon ship came, they refused to jump, even though the deck was soon almost vertical. "Everyone held on for dear life," recalled ensign Harlan Twible. "They were scared to go into that forbidding water."
They had good reason to be fearful, not just because of the sharks who swam to the site from miles away, attracted by the blood soon flowing from the dead and the wounded.
Many had not had time to find their lifejackets and very few liferafts could be found in the pandemonium which followed the Japanese onslaught.
Those men who did not drown immediately clung to debris or trod water to survive, finding respite only when the death of a crewmate made a lifejacket available.
For those for whom staying afloat was not an immediate concern, there were other dangers. The Indy's fuel tanks had ruptured and, as the men plummeted into the sea, many accidentally swallowed the oil surrounding them in great slicks.
After hours of vomiting, which left them horribly thirsty, they then faced the sun's relentless glare, which blistered their previously chilled flesh.
Their eyes burned, too, from the caustic saltwater that constantly splashed their faces and, as severe dehydration set in, many suffered hallucinations, thinking they could see a ship or island and excitedly trying to persuade the others that they should swim to safety with them.
"Their thrashing often attracted the sharks and we'd hear a bloodcurdling scream," remembered Edgar Harrell. 'Like a fishing float taken under the water, the helpless sailor quickly disappeared and then his mangled body would resurface moments later with only a portion of his torso remaining.
"That was then fought over by other sharks - a haunting sound and scene I cannot erase from my mind."
With men spread over thousands of square yards of ocean, Harrell found himself part of a group of about 80 frightened souls who attached themselves to each other's lifejackets so they could not drift away.
At one point, a brief spell of rain saw them opening their mouths heavenwards, eagerly swallowing what precious drops of fresh water they could catch.
By the second day, their group's number had already dwindled to about 40, many succumbing to salt poisoning when their extreme thirst drove them to gulp down mouthfuls of seawater instead.
"With our minds becoming unhinged, our tongues swollen and our throats squeezing shut, it's easy to understand why some of the survivors began drinking the saltwater," wrote Harrell.
"The boys who fell into this trap soon had violent fits, whooping and hollering and twisting around in the water with flailing arms. Suddenly, as if an explosion had taken place, they would fall into a coma and go limp. Sometimes this would happen in the middle of a ring of sharks."
To keep up morale, the men sang the naval hymn For Those In Peril On The Sea, and tried to convince themselves that help was on the way.
However, the speed with which the ship had sunk had allowed her to send only one SOS and that had been ignored by radio operators on other ships who, with the war so close to an end and the enemy facing defeat, thought it implausible the American fleet could have sustained such a blow. Instead, they decided the distress signal was a Japanese trick, designed to lure their ships into the area.
Neither does there seem to have been any proper procedure for checking whether the Indy had arrived in the Philippines, as scheduled, so the men remained at the mercy of their dead-eyed underwater predators.
The fact that the sharks tended to attack live victims close to the surface is consistent with them being oceanic whitetips (known as the Dark Knights of the Ocean).
Growing up to 10ft long, they are not the largest sharks, but their ferocity once led the oceanographer Jacques Cousteau to describe them as "the most dangerous of all sharks".
These killers were unused to retaliation and so they could sometimes be deterred by jabbing them in the eye.
The shipwrecked men also learned that there was safety in numbers, the sharks preferring to pick off only those who drifted to the perimeter of their groups. Mostly, there was little the sailors could do except pray they would not be the next victim.
"On numerous occasions, I recall seeing a large fin coming straight at me," wrote Edgar Harrell. "In horror, I would take what I thought would be my last breath and bend my knees up to my chest.
"Sometimes I could feel a fin brush my body. Other times I would merely feel the wake of the massive beast streaking through the water just underneath me.
"These gut-wrenching encounters caused me to feel as though I was constantly tied up in a knot and my abdominal muscles became completely exhausted, leaving my legs to dangle helplessly in the path of the mighty marauders."
Among the stranded men was the ship's chief medical officer, Dr Lewis Haynes, who became little more than a floating coroner.
"I'd look into a man's eyes and if his pupil was dilated and he didn't blink, I'd declare him dead," he said. "Then we would take off his lifejacket because we needed every damned one that we could get our hands on."
By the fourth day, even their lifejackets were giving up. Filled with the natural fibre kapok, they had long exceeded their buoyancy limit of 48 hours and therefore dragged many men beneath the surface.
Hopes of rescue had almost gone when the pilot of a U.S. bomber on an antisubmarine patrol looked down and saw the oil-soaked heads of men bobbing on the water.
By the next morning, the survivors' fifth in the water, destroyers were on the scene. No one knows how many deaths were due to sharks, rather than exposure or dehydration, but only 317 men remained alive. Many had excruciating infections from shark-bites and the attempts to clean the oil off saw many layers of decomposing skin peeling away with it.
For the U.S. Navy, the sinking of the Indianapolis was an embarrassment and details of what happened was not released until two weeks after the rescue and the day that Japan surrendered.
That triumphant news overshadowed the loss of the Indianapolis, but the Navy still sought to make a scapegoat of Captain McVay.
He was court-martialled for failing to follow a "zig-zag" course - a standard anti-submarine manoeuvre, but one he had no reason to adopt given the misleading intelligence he was given.
Demoted to a desk job, he was also sent hate mail by some relatives of the dead. This caused him more than 20 years of mental anguish. In 1968, he shot himself with his service revolver.
Captain McVay was finally exonerated in 2000, thanks to a final strange twist in the tale.
Three years previously, a 12-year-old Florida schoolboy called Hunter Scott began researching the sinking of the Indianapolis for a national history competition.
Talking to some of the 150 survivors, the tenacious youth eventually had enough evidence to persuade the U.S. Congress to clear Captain McVay's name.
Scott said he had been inspired by watching the 1975 movie Jaws, which featured the late Robert Shaw as Captain Quint, an Indianapolis survivor who went on to become a seasoned shark-hunter.
In a celebrated monologue, Quint relates his ordeal and eventual rescue. "You know, that was the time I was most frightened - waitin' for my turn [to be saved].
"Anyway," he concludes bleakly, "we delivered the bomb."