I'm not sure what people think would have been different if the Navy had identified and flagged the implosion earlier. They heard a sound. At the time they had no reason to suspect it was actually the Titan, which they did not know was missing and, to be fair, wasn't missing at that point. And...
I'm not a lawyer, but it might. You can't use a signed waiver as a get out of criminal negligence card.
I couldn't get you to sign a waiver, murder you, and then say "hey! they signed a waiver!"
It may be more a case that having anybody and everybody jumping in to search, at some point, becomes counter-productive. It's hard to listen for potential sounds in a pool full of rowdy teenagers.