There is in fact a German submarine sunk in the Potomac River. However,
it was not sunk there during the war, it was handed over to the US Navy
afterward. Then, as part of tests & exercises, it was sunk at least
twice. The last time it was sunk was off Piney Point in 1949, and is at
least 65 feet under the surface.

So, while it is not "marked as an obstruction" on any chart, and no
German sub ever patrolled Long Island Sound, nor entered the Potomac
River in search of Allied convoys, and it's for darn sure no IJN
submarine ever shelled Seattle.... there *is* a U-boat sunk in the
Chesapeake!

https://www.denix.osd.mil/denix/Publ.../panther2.html

The truth is out there!