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I like the idea behind the Super Falcon model: fixed positive buoyancy with an inverted wing design.

Like a plane, but with gravity replaced by buoyancy and the Y axis inverted so the wings "lift" you down.

But what if you want to stop and contemplate something? Marine fauna will probably flee from moving objects.

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

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

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.

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