Why are submarines called submarines?

Question

How did submarines get their name?

Answer 1

Because if they were called "FRED", then nobody would understand what they were talking about.

Answer 2

Marine being water. Sub meaning below. Below the water.

Answer 3

Marine is - pertaining to the sea

sub is - below

Below sea - submarine

Answer 4

Because U-boat was already taken.

Answer 5

Because the Navy would not support the name

"Ships that kill whatever ship it wants, whenever it wants, and a whole bunch of other stuff too"

Answer 6

because if you called them a truck,that would be silly

Answer 7

What else would submariners serve on?