I think the way we were discussing it here, the waiver is not "null and void in its entirety," it's just ineffective as against a third party action for wrongful death in the stated circumstances. Since (if I remember correctly) "wrongful death" usually requires some form of actual or implied scienter (recklessness or extreme negligence) it can not be waived..
On the "unable to waive rights of third parties," people do that all the time - look at a settlement agreement, or other simple contract. These instruments are effective to waive an action by the party executing and (usually) their heirs, assigns, executors, and etc....wait hold on...ok, I thought I had a clause on my computer here at home, but (thank God) I don't; maybe when I get to the office.
In the end, there's differences to be discussed with waiving a cause of action because (a) the diver died of a heart attack while diving; (b) the diver died and was 3/4s (or 50% or even 25%) at fault and the remainder was due to the simple negligence of the dive operation; (c) death due to 100% simple negligence of dive op; (d) death due to recklessness of dive op; (e) death due to affirmative misconduct of the dive op (ie, DM turns off someones air at depth as a joke and they are scared to death)...there are also other levels of "conduct," and the determination of which level you are on will determine how far and wide the waiver previously executed by the decedent will go...this is, of course, a simple analysis, as there is not enough room here to fully and completely discuss the issue which could take up 1000s of pages...
You'll also find that there certainly is rarely an instance where you can make a sweeping or broad-stroke statement about applicable legal rules, which are heavily fact-dependent in their application...
If you look, the Ginnie waiver applies to risks inherent in engaging in the "Activities" (scuba diving); it doesn't say anything about waiving the negligence, or recklessness or intentional conduct, of anyone, which are not included in the definition of "Activities" (nor would you expect that they were "inherent" in the Activities) and therefore, I believe that it would be a pretty strong argument that the waiver was inapplicable thereto...I also believe that you'd at least make it past summary judgment on the question...