USSR; US; UK; Germany & Japan.
World War I submarines were primarily used to locate and attack surface cargo and warships.
Germany used U boats.
submarines were used in WW1 and the weapons used were torpedos and a deck gun.
They were used extensively throughout the war - from beginning to end.
They were used by submarines to navigate when not in open waters, They were also used by submarines and anti-submarine ships to locate enemy submarines.
During World War 2, it is reported that over 2,000 variants of Japanese Submarines were used during World War 2. Out of this, it is estimated that 10,000s were likely to be used during WW2.
Germany
Yes, many submarines were used in WW1 by most of the major combatants.
World War 1
Germany, they were submarines.
Why don't you just Google "World War 1 Submarines?"