On American public television, the 3rd season of the series 'Nazi Mega Weapons', has an episode on Germany's submarines of World War II. It serves pretty well as a primer on u-boats and the Battle of the Atlantic.

By early 1943 Nazi Germany is losing the battle in the Atlantic. Their U-Boat menace is no longer invulnerable to attack; it could cost them the war. But Hitler and Kriegsmarine High Command are one step ahead. Since 1940, they’ve been planning a submarine capable of fast underwater speeds and phenomenal firepower that will outperform anything that has gone before. Designated the Type 21, it’s the world’s most advanced submarine: Hitler’s sea-faring super weapon.

Check you local PBS television listings or you can watch online at:
Nazi Mega Weapons