The type VIIC, which was the most common u-boat of the war, had a maximum safe depth between 185 and 250 meters. It would be crushed at about 300 meters, possibly deeper.
A WWII U-boat could clear the surface in 30 seconds.
1,160 U-boats were built by Germany during the 2nd world war. The prefix U in U-boat stands for "Unterseeboot", a German term meaning simply...