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Each time they go down they have to jettison some waste to return to the surface?

I'm no sub nerd but it seems obvious that unless you can otherwise alter your buoyancy somehow, your only option is to drop weights.

I think submarines use ballast tanks they flood with sea water and empty with pumps to vary their buoyancy. Titan had none of that complexity AIUI, and multiple articles I read mentioned dropping "ascent weights".

Ballast tanks don't work at these depths because of the extreme pressure, which is why all these deep sea vehicles are submersibles rather than submarines. In fact I think if a submarine goes below a certain depth then it can't adjust its buoyancy to return to the surface again, but I am also not a sub nerd.

You can use compressed air. The COPV Spacex uses for tank pressurization are rated higher than the Titanic depth. And as depth increases the differential decreases.

So you really only care about their structural integrity at the surface.

Which would be an actual appropriate application for carbon composite...

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