Reply #3: Subs could spot ships silhouetted against lights along the coast

[Hobarticus]

Since America wasn't at war, the Kreigsmarine had little fear from sub-hunter warships, so U-boat crews would surface at night off large cities and ports and just watch and wait.

As a ship approached between themselves and the coast, they could make out the speed and course by the silhouette of the ships. Churchill implored the States to start enforcing blackouts for that very reason.

Successful sub operations at that time were still largely based on good intelligence, ie. knowing where a ship was going to be at such a time, patience, and good luck. If you can sit off the coast of a country along known shipping routes, it's pretty much a turkey shoot. That period was known as the "happy time" for German submariners; they had terrific success because the US didn't take the threat seriously.

Ships were sunk in waters so shallow, the superstructure would still be above the waterline when the keel hit bottom.