That's a good question and I learned some stuff...That's a cool video, Spec.
What I don't get is torpedos. I saw a cutaway version of a German U-Boat torpedo at a museum in Chicago and it's just a tube packed completely full of propulsion equipment and explosives. It has no "fins" or "wings" to make it "fly" through the water and there's no way there's enough space/air in it to displace enough water to make it "float."
I don't see how the things don't just sink as soon as they're ejected from a ship or submarine, no matter how much thrust they have.
Edit: Apparently the thrust and small fins are enough to keep it going: lose the thrust and it sinks.