Most six packs aren't going to be liveaboards or even overnight boats which eliminates a lot of potential boat issues. Most operators take care of their essential equipment on their boats, because if they don't they are out of business.
Let's put this into some perspective. The OP is highlighting some rare instances, one of which happened years ago. There's inherent risk with any vessel on the water. I've seen stories of brand new $1 million dollar vessels fail, but again it's rare. Tens of thousands of scuba and fishing charter six pack trips are made every year and very few sink, if any at all. I don't have the actual data, but if you forced me to guess, I'd say 99.9% of the US trips are successful. I like those numbers and it's probably why they aren't inspected vessels. It's not something that would be on my radar to worry about. Talk to the operator or captain about the boat's condition if it concerns you. They all should go over safety in the pre trip briefing.
If you're a boat diver just buy a PLB, put it in a canister and you've virtually eliminated the lost at sea issue, which is a greater risk than being stuck in a cabin on a six pack as it's sinking. Recognize with living comes some level of risk. We can follow every godforsaken rule and regulation and things can and will go sideways from time to time. Do what you can to mitigate it and have fun, live your life.