Answer:
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 "undersea boat". By the end of the war, only about 360 survived with German submariners suffering one the highest death rates of the conflict. Typically, a German u-boat of WWII had a surface speed of about 17 knots and a submerged speed that ranged between 4-7 knots. They had normally operated at a maximum depth of 200 to 250 feet, but in an emergency could go as deep as 400 before being crushed by water pressure.