"...for it was the weather that was the most violent enemy of all. For eight days they steamed straight into a westerly gale: five hundred miles at a grindingly slow pace, buffeting through a weight of wind that seemed to have a personal spite in every blow it dealt." -Nicholas Monsarrat, The Cruel Sea.
It was the logistics train that saved Europe. From 1939-1945, lightly armed, newly built Liberty Ships and an array of Allied (mostly British and Commonwealth) naval vessels with air support constituted the "sinews of war" that brought gas, guns, food, ammunition and everything else a ground army needs to win, to the shores of Britain, Africa, North Russia and then later, continental Europe.
German Admiral Karl Donitz identified the logistics lines across the North Atlantic as the center of gravity for the Allied war effort against the Third Reich. He then set about to sever them. U-Boats hunted the Allied merchant ships in groups called Wolf Packs and sent 36,200 Allied civilian and military sailors to their deaths. My Great-Uncle (an engine room snipe) was one of them. U-482 put two torpedoes into the side of his bulk-fuel carrier, the SS Jacksonville, 50 miles north of Londonderry. Two men survived, only because they were topside.
But the U-Boats weren't the only enemy. The weather and sea conditions also tormented the Allied crews as we can see in another passage from The Cruel Sea (whose author lived it, by the way):
"Aboard Compass Rose conditions were indescribable. She rolled furiously, with a tireless malice allowing no rest for anyone. Cooking was impossible...Everything was wet through: some water had come down a ventilator and flooded the wardroom: forward, the mess-decks were a crowded hell of saturated clothes, spare gear washing around their feet, food overturned- and all the time the noise, the groaning slamming violence of a small ship fighting a monstrous sea. Compass Rose caught in a storm that could take hold of her bodily and shake her till the very rivets loosened: a storm that raged and screamed at her until they were in the shelter of land again: Compass Rose adrift on a malignant ocean, seemed doomed to ride it together."
The Cruel Sea is full of passages like that. U-Boats aside, it is a testament to the skill and the emotional endurance of those sailors that they went through horrendous conditions like that, day in and day out for 6 years, all the while being hunted by submariners of the German war machine.
Technological advances made dominance in the Battle of the Atlantic go back and forth. Radar and sonar were a great boon to the Allies, but then the German schnorkel and the acoustic torpedo swung the balance back momentarily. In the end, it was the compromise of the German Enigma code that many historians credit with giving the Allies the best success in hunting, finding and killing German submarines. There were a plethora of other stratagems such as the convoy system and the coordination of air and sea power that drove the rest of the nails into the coffin of the U-Boat service of the Kreigsmarine.
But again, I think we have to give the most credit to human endurance in the face of "wrath of god"-style weather that enabled the Allies to succeed in the Battle of the Atlantic.
"...he...felt the gale whipping and tearing at his face...Compass Rose lurching under his feet as if the world itself were drunk, it was with a body from which every instinct save dumb endurance had been drained...the water crashed and thudded against their side, and the wind howled at them out of the blackness as if it had a conscious intention of terror. Round them there was nothing but a waste of sea, a livid grey whipped up here and there to white foam; and then beyond it, like a threatening wall, the surrounding dark, the chaos and flurry of the night."