I posted this awhile back, but due to many sstill not believing that U.S. Subs had decoys I will repost.
I remember a while back people were saying that American subs did not have decoys, well that is just not true, after spending my summer volunteering at the U.S.S. Batfish I learned that American subs used alka seltzer and shot it out the signal flare ejectorin the aft torp. room, and they also used a fake periscope that was metal and it floated on the surface it was about 3 feet long and was fired the same way as the alka seltzers. The alka seltzer was meant to sound like a propeller swishing in the water, this is what is modeled in the game.
the first paragraph in this link lists the decoy capabilities:
http://www.usscod.org/ator.html