It was June 5, 1944, and the Glasgow boy, not long turned 20, was making his final preparations.
Men's lives would depend on his skill and judgement.
Come dawn, he would be part of the biggest seaborne invasion the world had ever seen.
There were nearly 5000 landing ships and assault craft escorted by six battleships, 23 cruisers, 104 destroyers and 152 escort vessels, as well as 227 minesweepers.
Its task: to destroy the German coastal defences in Normandy, and to enable Allied forces to begin the task of routing the enemy from occupied western Europe.
Its name: Operation Overlord.
June 6 was D-Day.
The plan was for a 1200-aircraft assault behind the invasion beaches after midnight, followed by a massive amphibious landing of 175,000 troops from Britain, America, Canada and other Allied countries on five Normandy beaches, code-named Utah, Omaha, Gold, Juno and Sword.
John, from Cambuslang, had left school early. By 15 he was working in Glasgow's meat market. At 18, he signed up for the Royal Marines, and became a corporal.
"I already had a relative who was in the Corps, and I asked to go into the Marines," he says in his room at veterans' care home Erskine Home.
"There were three depots in those days, one of which was Chatham [in Kent]. My number was Chatham X113326." He smiles. "My memory's not bad, is it?"
Juno Beach, which was his target, was to be stormed by the 3rd Canadian Infantry Division, whose task was to seize the German airfield at Caen-Carpiquet.
Men from 48 Royal Marine Commando were to land at Saint Aubin- sur-Mer and capture the coastal defences at Langrune-sur-Mer.
"I was the coxswain on an LCM, a Landing Craft Mechanised, mark III, to be technical," he says. "That's the one you see in a lot of war movies, the ones with the big ramp that opens. They can take 100 men - they were pretty big - or they could take a light artillery piece.
"At a push, if the men were stacked in, they could take about 150. I took about 130 in the first wave. And there were six crew, including myself.
"My job was to get Canadian soldiers onto the beach at Bernieres-sur-Mer. I picked them up on the south coast when it was still dark and took them across the Channel.
"You want to know what the worst part was? The worst part was when the Germans were retaliating by firing mortar bombs, which went high into the air. You can imagine one of these exploding inside a landing craft. That was one of our worries."
The crossing that day was rough. Countless soldiers were violently sick as their craft battered through the waves.
Even now, John is proud that his Canadians barely got their feet wet. The ramp was lowered, they poured forth, and immediately had to confront the German defences. It was no easy task.
The beaches and the area beyond had everything from concrete and steel pillboxes to hidden machine-gun nests and artillery positions.
Years later, one Scots-born veteran of Juno spoke vividly of his memories.
"It was the noise, the screams, the bodies littering the beach and the overwhelming sense of helplessness," he said.
"My driver, who was from Dundee, was killed on the beach. He was just 24."
Of the 14,000 Canadians who landed at Normandy that day, 340 were killed, 574 were wounded, and 47 taken prisoner.
Guns blasting, shells exploding, men screaming, men dying. John couldn't help but be aware of the chaos and the noise, "but we had to be focused on our job, of getting the men onto the beach. Our training came into effect. We had to be disciplined about it.
"What I remember isn't so much the fact that people were shooting at you. It was the noise of the British battleships, which were shelling the beach. And the Germans were defending their positions. Shells were bursting all around us. We were caught in the middle."
There was, as he recalls, no let-up. "We landed lots of men on the first day or two and once that was done, we delivered supplies and ammunition. I was there for about a month, all told."
By the end of that momentous day, the Allies knew that not all of their objectives had been achieved - but at least their forces were firmly established on Normandy.
John found himself in another theatre of war, in Burma. He celebrated his 21st birthday in Rangoon, and wouldn't finally come home until September 1946.
Inbetween Normandy and Burma, he married Helen, whom he'd met one day in the market where he worked. They would go on to have two daughters, Liz and Barbara.
In his room at Erskine, we look through some of his memorabilia. There's the Normandy campaign medal, commissioned by the Normandy Veterans' Association.
There's a picture of his landing craft. And there's a cutting from the Evening Times - a letter he had printed in 1994, on the 50th anniversary of D-Day.
"As a 20-year-old corporal coxswain in 651 L.C.M. Flotilla, Royal Marines, I helped to land Canadian troops on Juno Beach on D-Day," it says.
"We can all hold our heads up proudly and say 'I was there' and bow our heads sadly when we remember our fallen comrades."