In February 1915, a submarine U-6 (Lepsius) was rammed and both periscopes were destroyed off Beachy Head by the collier SS Thordis commanded by Captain John Bell RNR after firing a torpedo.
On 7 May 1915, SM U-20 sank the liner RMS Lusitania. The sinking claimed 1,198 lives, 128 of them American civilians, and the attack of this unarmed civilian ship deeply shocked the Allies. According to the ship's manifest, Lusitania was carrying military cargo, though none of this information was relayed to the citizens of Britain and the United States who thought that the ship contained no ammunition or military weaponry whatsoever and it was an act of brutal murder. Munitions that it carried were thousands of crates full of ammunition for rifles, 3-inch artillery shells, and also various other standard ammunition used by infantry. The sinking of the Lusitania was widely used as propaganda against the German Empire and caused greater support for the war effort. A widespread reaction in the U. S was not seen until the sinking of the ferry SS Sussex. The sinking occurred in 1915 and the United States entered the war in 1917.
The first submarine built in Germany, the three-man Brandtaucher , sank to the bottom of Kiel harbor on 1 February 1851 during a test dive.  
The inventor and engineer Wilhelm Bauer had designed this vessel in 1850, and Schweffel & Howaldt constructed it in Kiel . Dredging operations in 1887 rediscovered Brandtaucher ; it was later raised and put on historical display in Germany.
The Sinking of SS Leopoldville
One of the biggest naval disasters ever to happen to American forces took place on Dec 24, 1944 off Cherburg, France. Read all about it.
The Fight Back order of 1943
This very interesting page explains the controversial order of 1 May, 1943 to stay up and fight the aircraft attacking the U-boats.