Actually bigmouth_strikes, there is one important thing you left out. I'm no expert, so I will screw up all the terminology.

I believe that the ballast is less important to the climbing and diving than motion and the "wings" or whatever they call the fins that stick out of the top conn tower and the ones in the back. Basically the submarine is a lot more like an airplane than like is a diving chamber. As it moves through the water, the fins/wings generate lift, just as an airplane's wings do. Since water is much more dense than air (about 826 times under normal circumstances), you only need itty-bitty wings to generate lift.

Thus, a submarine which is zooming around underwater and suddenly loses power is only a little bit less screwed than an airplane. They do have compressed air which they can use for an emergency surface maneuver, but from what I gather, this is not a pleasant thing.

Update: 5 August 2005. Yesterday a Russian mini-submarine sank after its propeller was entangled by a fishing net.\

Also The Custodian, something may have positive buoyancy at the surface, but still sink at lower depths. Net buoyancy is a function of water pressure; at lower depths, the increased pressure will overwhelm the buoyancy. It's a lot harder to float with 500 feet of water sitting on top of you than when you're at the surface. Take the human body. Bodies not wearing cement shoes float under ordinary circumstances. However, world record holders and seekers for free diving (without air tanks) pass the point of neutral buoyancy and must actively swim upward, or they will sink. I read a great article on these guys in The Atlantic Monthly a few years ago (it looks like it was the May 1997 issue).