Well, German Submarines reached all the US east coast, from Florida in the south to Maine in the North during operation Drumbeat. Actually they ranged further south than that, well into the Caribbean but you did ask about the US coast specifically.

During this period (Roughly the first half of 1942) German submarines sank ~600 ships, primarily in the American theater (US east coast to about 200 miles off shore). So, I think we can safely say that there were more than just close calls along the US east coast. The Kriegsmarine lost just 22 submarines in the same operation and if memory serves only 20 of those were due to enemy action (the last 2 simply being missing, lost either to accidents, mines or any of the 100 other things that can go wrong on a Type VII or Type IX submarine).

At the time the US was surprisingly unprepared in terms of submarine counter measures. The USAAF was neither well trained nor well equipped for anti-submarine operations and the USN was equally unprepared with relatively few ships in the area suitable for convoy escort operations or chasing submarines and what ships they had were not properly utilized.

The RN had plenty of experience protecting ships against submarines from the first 2,5 years of the war and were happy to share it with the USN. Measures like frequent daytime air patrols to restrict submarine mobility, ships sailing in convoy whether it was escorted or not, coastal blackouts etc. The US implemented precisely none of these measures and as a result the German Submarines were very successful. U-123 under Reinhard Hardegen (The submariner, not the SS officer) for example sunk a tanker right outside Long Island. Like, within sight of Long Island. There were about a dozen destroyers in the Harbour in New York at the time. None of them were sent out so the U-123 was allowed to continue sinking ships around NY despite it operating in dangerously shallow waters with little ability to hide and evade enemy action.

So how close did U-boats come to the US East Coast? Within spitting distance. Repeatedly. For months.

Now, you’ve gotten a good answer about the Second World War, I’d like to add some detail regarding the First World War! German submarines actually prowled along the Eastern Seaboard during the conflict, with two major phases.

Before the US Entered the War

The first was before the United States was a belligerent. In 1916 the United States was visited twice by a merchant submarine called the Deutschland. Her first visit was to Baltimore, Maryland, and her second visit was to New London, Connecticut. Her sister ship, the Bremen was supposed to have visited New London, but had been lost at sea without a trace and so the Deutschland made a second trip. These two submarines were large with capacity for some dry cargoes inside the boat, as well as “wet cargo” which was stored between the internal pressure hell and the outer hull.

These two vessels, while having a nominally peaceful mission, were partly the brainchild of Germans arm manufacturers and the German government. The British blockade prevented the German government from receiving war material such as copper and nickel. Those materials were useful in creating things such as cannons. Before the outbreak of war, the German government and German companies had purchases materials such as that from American companies, but the outbreak of war prevented them from being shipped to the United States. The merchant submarine, a vessel that could evade the Blockade by diving, was thus seen as the solution. The merchant submarines were at least nominally owned and operated by the newly created Deutsche Ozean Reederei GmbH, or “German Ocean Navigation Company”. This was founded by representatives of Norddeutsche Lloyd and Deutsche Bank, and was run by board members of both – all of whom had ties to the Etappendienst, a secret logistical service for the German Navy. The Imperial German Navy provided construction labor, material, diesel engines, and the crew for the Deutschland and Bremen. The crewmembers selected were already experienced submariners, and had often been employees of Norddeutscher Lloyd prior to the war. Officially, of course, the German government did not admit to this. The vessel had to be made to seem entirely as if it were a private merchant venture.

The Eastern Forwarding Company, founded by Henry G. Hilken and Paul H. L. Hilken, a father and son team who were agents of Norddeutsche Lloyd, was a front company for the German government. They had been asked to create this company by the Etappendienst, this was to help give some distance between the German government and the materials traded onboard these Merchant submarines. The EFC purchased their own tug-boat to assist with docking the Deutschland and Bremen, warehouses and dock space in Baltimore and New London, as well as hired their own stevedores for the loading and unloading of cargo. They specifically hired black stevedores as they assumed that they would not be spies for the British.

The Deutschland arrived in Baltimore, Maryland on July 9th, 1916. While not everyone was enthused, such as Submarine designer Simon Lake who accused the Deutschland of violating some of his patents, the event was generally used as a PR stunt for the Germans. Some of the crewmembers even made an unauthorized trip to Washington D.C., where they were arrested for speeding but fairly quickly paid their $20 bond. The crew otherwise met President Wilson, had their photograph taken whenever possible, and generally spent their time meeting with various German groups in the United States, that is until the crew was abruptly kept on board. The Historian Dwight Messimer argues that the Deutschland had actually taken gold onboard that had been from the interned ship SS Kronprinzessin Cecilie. While this story may or may not be true, the crew was kept onboard July 19th until August 1st. On the whole, their visit to Baltimore was largely successful and proved to be a PR boost.

The next German submarine to arrive in the United States was supposed to have been the Bremen, but as mentioned earlier she was lost without a trace. Thus, the next submarine was actually U-53 captained by Hans Rose. On October 7th, 1916 U-53 was spotted by the American submarine D-3 sailing towards Newport. U-53 was allowed to be in port, and Hans Rose claimed they needed no assistance with fuel or supplies, as well, American officers were invited to inspect the boat. By 6pm that evening the U-53 left Newport, and once leaving American territorial waters, sunk five vessels – 3 British, 1 Dutch, and 1 Norwegian. Submarine warfare had come to American shores.

The Deutschland returned to the United States on November 1st, 1916 in New London, Connecticut. New London was an important port for submarines. Just up the Thames river sat a brand new Submarine base (which still exists today) and just across the river the Electric Boat Company was (and still is today) headquartered. This visit was a little rockier for the crew. U-53’s actions only weeks earlier had left a mark and people were much more cautious about the Deutschland on its second visit. For example, local white stevedores were angry that the Deutschland had its own complement of black stevedores to load and unload cargo. Crewmembers also got into a bar fight which resulted in slash wounds to an American. But, the arrival started off fairly similarly to that in Baltimore, with some eager crowds and interested persons. The Deutschland was inspected by Yates Stirling Jr, commander of the US Submarine Base who wrote a report on the boat and its specifications, design, and capabilities.

The New London Day, the local newspaper, ran many stories in the coming weeks on the Deutschland. Companies even used the occasion to advertise. For example, an insurance firm, Meyers and Hollandersky, took out a newspaper ad which looked suspiciously like an article. It led with details of the boat’s arrival, but then segued into an ad. It was the talk of the town, yet, while there were still tourists and other interested parties who wanted to see the boat, the same kind of enthusiasm that was seen in Baltimore had almost entirely disappeared – U-53 had ensured that.

On November 17th, 1916 the Deutschland attempted to leave New London at night. As they approached the “Race” and “Race Rock”, the part of the Long Island Sound which meets the Atlantic Ocean, they collided with a local tugboat which was towing them out to sea, the T. A. Scott Jr. The American crew of the tugboat was killed, and the Deutschland damaged. After returning to port the Deutschland ended up facing legal suits over the crash. These were actually settled out of court in August 1917, after the United States and Germany were at war. In the meantime, the Deutschland was repaired and left New London on November 21st, where she was actually filmed leaving.