[amelius]

Why not zero buoyancy, and using some kind of expanding airbag for emergency floatation?

[jleahy]

Typically people would use drop weights for emergency floatation, as that's more reliable.

[dTal]

The CSS Hunley used this design, to no avail. It's not clear what went wrong but I think the larger point to draw is that such a system requires positive action, with more things to go wrong; it is "fail-deadly". Positive buoyancy and dive planes are "fail-safe" - you stop, you float.