The problem being that those U-Boats cannot conceivably knock the US out of the war, unlike the UK. At this time, the US can get anything and everything its economy, people, and warmaking ability need from North America or, in a few minor cases, the Americas. So for a few months, maybe a year at most, the Kriegsmarine owns the sea near the US east coast... and the US ships things by land while cultivating alternate sources of supply.
My mini-TL:
The US suffers some minor, short-term economic dislocation, nothing which is allowed to affect its build-up to fight Germany and Japan, and recovers fully within at most a year. By that time, the USN and USAAF, as well as the civil authorities, have gained the experience in coastal ASW warfare that they so desperately needed to kick the crap out of Germany's U-Boat force.
The tide turns sometime in mid-42, give or take a few months, and the Kriegsmarine gives the orders withdrawing the U-Boats from US coastal waters after taking near-catastrophic losses. A much-winnowed Kriegsmarine begins heavily patrolling the Atlantic, waiting for the US to go on the offensive.
Taking a time-out in the Atlantic to hit Japan, the US moves to link up with the heavily-garrisoned and still resisting Philippines, in cooperation with the RN and RAN. This bears fruit; the Japanese can't allow American or ANZAC reinforcements to reach the Philippines, or it will further imperil their advance into the East Indies, and eventually into New Guinea. The IJN is obliged to meet the US and RN in battle; combined, the two manage to concentrate more airpower, helped by American landbased aircraft in the Philippines, sinking three IJN carriers for the loss of one. In a side effort, the Americans begin to take back the Aleutians.
By mid-1943, the Americans have assembled a massive naval task force with more ASW support than ever seen before. The objective: to poke the European Axis in the eye with a very large, white hot iron poker. Something they can't possibly ignore; bombing the living daylights out of crucial port facilities in northern Germany and western France. Hitting the U-Boat force where it lives is the one thing guaranteed to enrage Hitler and the Nazi elite, especially with the setbacks on the Soviet front.
The writing is on the wall; the Axis are losing. Having failed to take the Philippines, Japan is gradually being forced out of the East Indies by partisan forces heavily supplied by the British, Australians, and Americans. The Japanese lines of supply are no match; with the Philippines as a base, the USN can sortie and punch out Japanese supply ships almost at will. Additionally, the US learned well in the battle of the Atlantic; its submarines' effectiveness is greatly improved. Breaking Japanese codes has allowed it to position subs to sink two more carriers, though anything further will start to look suspicious.
In late '43, the British declare war on the Third Reich, having decided (in conjunction with the US), that allowing the Soviets to conquer the entire Nazi edifice would be less than ideal.
Need to sleep, can't continue writing... summary of the rest is as follows: European war proceeds more or less as in OTL; D-Day is delayed a few months, and American and British armored units are slower, without the OTL experience in North Africa. Austria, as a result, becomes the People's Republic of Austria. East Germany is a bit bigger ITTL, as well. Without being conquered, Italy signs a separate peace after Allied forces cross the Rhine, turning on Germany and tying down German forces in the Alps. In the Pacific, the Japanese are confined to the Home Islands, Manchukuo and the Korean Peninsula by late '44, because the Philippines offer a wonderful location from which to crush efforts to keep Indochina and China proper supplied. American support for partisans in both areas have the IJA running scared by then. War ends as in OTL, with butterflies changing the targets, because an invasion continues to be a quick path to killing 500,000 American soldiers and 4 million Japanese civilians.