I am aware that U-boats sunk at least one ship in Casco Bay, Maine. I don’t know if it is true, but legend has it that a sunken U-boat also lies at the bottom of the bay. Was there any fighting either on land or sea near the US east coast?
In the early part of the war there was no convoy system up or down the East coast. And the city lights along the coast were not dimmed outlining merchants against the coast for U boats. It was the return of the Happy Time for the U boat captains.
There was also Operation Pastorius, a rather slapstick German attempt to sabotage American war efforts using agents dropped off in New York and Florida.
Hitler’s declaration of war upon the US is often looked upon with bewilderment as he was under no obligation to and it removed the onus of doing so from Roosevelt. Hitler actually had no reason not to; the US and Germany had been in an undeclared naval war in the Atlantic since late summer '41 and full scale war with the US was clearly coming. By taking the gloves off the U-boats were able to sink 1/4 of all of the shipping sunk by U-boats during the entire war in the span of 8 months at a negligable cost to themselves. Add to that Hitler grossly underestimated the US.