I read The Good Shepherd and loved it, and the movie is a mostly faithful adaptation. I didn't mind the minor differences (no spoilers).
I am saddened to see some people don't realize the United States was heavily involved in escorting convoys and hunting u-boats during World War 2.
Thousands of Americans lost their lives in the Battle of the Atlantic.
Over 3,500 merchant ships were sunk, with over 1,500 of these ships belonging to the US Merchant Marine. The sailors of the US Navy also suffered, with quite a few US destoyers sunk or damaged during the the Battle of the Atlantic. The first of these American destroyers sunk was USS Reuben James DD-245 (more than a month before Germany declared war on the US).
The last US destroyers sunk by u-boats during the height of the Battle of the Atlantic (1939 to 1943) were USS Bristol DD-453, USS Borie DD-215, and USS Leary DD-158 (in October, November, and December of 1943, respectively). More US destroyers would be lost before the end of the war.
Other allied nations also suffered during the Battle of the Atlantic, especially England and Canada.