And the ballast is held on via electromagnets. Lose (or deliberately cut) power, you're going up.

I don't think it is on this "sub". I believe I read they used a material that degraded over time in water.

Isn't this like a huge waste of energy?

I don't think weight is a huge issue for submarines (unlike say airplanes), so taking a few batteries along should be fine.

(I don't know anything about subs) you could even store a few batteries in the ballast.

Between "waste some power on a magnet" and "we're permanently stuck on the bottom of the ocean about to run out of air", I know which one I'd pick.

There's a difference between a "waste of energy" and a failsafe emergency system that happens to use a lot of energy.

It's nothing compared to amount of power to the engines, you don't need kilowatts to "just" hold a ballast

Clear tradeoff for safety.

Sure, but they can also incorrectly steer themselves into wreckage that causes them to be stuck...

Well, you need to be able to change the weight of the capsule somehow, otherwise you'd be stuck bobbing at the surface. (Or at the seabed).

The puzzling bit, from what I have gathered, is that the ballast is not fail safe.