PLB's last for 24+ hours. They're guaranteed. That's one of the reasons they limit you to a certain number of GPS tests before they say you need to send it in for a battery replacement. 24 hours when they KNOW where you are is a long time. Unless you're literally in the middle of the ocean, chances are somebody can get to you within 24 hours, even longer if you intermittently transmit. HOWEVER, I'd be reluctant to turn it off after a couple hours, since the first 24 hours are going to be the most important in terms of finding you.
Hypothetically speaking, IF Cameron made it to the surface, and IF he had a PLB, and IF he activated it, based on the cell phone coverage around the island, the command center could have literally talked a boat driver to his location on a cell phone.
I think we can all safely assume that there is SOME common sense involved when making a judgement call as to whether or not to carry something. I get it, we all want to be obtuse and whine about every exception to every situation, just like other people seek to shoehorn every situation into a finite rule. But the fact of the matter is the ocean is unpredictable as hell, and if there's a solution that allows us to mitigate some of that uncertainty, making lame excuses not to use that solution is silly. And for the record, if I were diving Lake Powell, I'd sure as hell take my PLB. It's huge, and there are a lot of places where there is zero traffic.