Lindsey's mini-sub battle with Coffey. He's trying to kill her husband before he can get back to the diving rig, so she proves that Ramming Always Works and slams into the other sub until it shorts out the engine, then knocks it over the edge of the trench, where the increasing pressure literally implodes the front window.
Her bloodthirsty expression just makes it more awesome. You could get the impression she was just waiting for an excuse to get that guy.
Some of the more practical stunts and effects, such as the breathing liquid scene; it's easy to imagine Ed Harris's character's panic as he has to willingly let his helmet fill with the fluid while the crew tries to keep him calm.