My understanding is they're relying on inertial navigation and water pressure sensor (to know the current depth) - you could turn on active sonar to map the sea floor, but that kind of ruins the stealth thing, and you can't look out a window. If there's a submerged obstacle that isn't mapped, especially another sub, you won't know it's there until it's too late. Which is pretty much what happened in the HMS Vanguard and Le Triomphant collision.