I actually learned the answer to this one this week, so I figured I'd share, because it's pretty cool.
First, a quick lesson on torpedoes:
Torpedoes are aimed to hit where on a surface ship? a) The side, b) the bow or stern, or c) underneath, missing the ship ?
The correct answer is c). Detonating a torp underneath the ship creates a gas bubble, and the ship rides up on it a little...and then snaps in half. Military ships are huge things that cannot possibly support themselves under their own weight. They are built on huge struts that are usually cut out by experienced divers with torches after it's sitting in a flooded wet dock.
So when Lelouch sent up a massive air bubble, he actually sunk all those ships at the same time when they snapped in two under their own weight.
Here's a video of a destroyer being hit properly with a torpedo:
http://www.metacafe.com/watch/124812...ts_battleship/
Submarines are a different matter entirely. Water gets in
anywhere, the boat sinks.