I just love these old school visualizations: here are all the ships lost by the British Navy during World War II. Many of these were hunt down by Nazi U-Boats. It's a staggering amount but it could have been a lot worse.
Thankfully, the good guys cracked the Nazi Enigma encrypting machine and, as a result, the Allied neutralized the Kriegsmarine's submarine fleet. [Longstreet — Thanks Jim!]