As I recall, in Connecticut and maybe in other places, even with a signed waiver, you can't contract away liability for negligence unless an industry like the ski industry gets the legislature to limit their liability.
I doubt this deep sea dive company would have had some special law written for them, where the waiver would protect them if sued for negligence. Big problem here is that without recovering the submersible, how do you prove negligence? Unless, the design of the submersible violated the standards of its industry.
Hope somehow they can miraculously rescue these passengers to make this speculation moot.