If you know someone has dangerous behavior and is going to really hurt (or kill ) themselves sooner or lafter, you hope they only hurt themselves when they do something really stupid, not taking out others.
What's really dangerous? For many it's the fact that they don't dive in the same way. I can't think of the last person I wouldn't dive with. I just adapt.
My simplified buddy check is two fold:
- Do they look like a diver?
- Did I hear them breathe on their equipment?
For the record, I teach my students to breathe on their regs while watching their SPG. If I hear them breathe, I assume they were checking their SPG. If we're diving together for the first time, I'm going to ask if you did just that and make sure you did. If I didn't hear you breathe, I'll just ask you to do just that. Pretty simple and very effective. If they splash without a piece of gear and their air is on, what's the big deal? Sure, it's embarrassing, but how dangerous is it to jump in without a piece of gear if you can still breathe? You mentioned that someone sank like a rock, well that's a weighting issue. Everyone should have been taught how to inflate their BC orally, so they should be able to stop their descent pretty easily if their air is on. Yeah, get rid of the extra weight but this should be an easy fix, even at depth. No mask? Well, you can still breathe, right? Breathing is the real biggie. Everything else is resolvable.
Now here's a reality check. How many people know what their buddy's tank pressure is DURING the dive? Do you know how to extrapolate their consumption based on your own? I've heard people complain bitterly about their buddy running out of air and my first thought is "Why did you allow that to happen?" I'm my buddy's redundant brain as well as their redundant air, eyes, buoyancy etc. Why dive ignorant when you can know? You can't make timely decisions without that information. It's my opinion that you're not much of a buddy if you don't know.
To be clear, I dive like my buddy is an "SOB" or Same Ocean Buddy. I don't expect them to realize if/when I'm in distress and that alone affects how I dive. That doesn't mean I have to be an SOB and so I take any buddy commitment seriously. Sure, if I am very comfortable with you, we might decide to be functionally solo divers, while saying we're buddies, but that's reserved for very, very few. But just because you will probably ignore me, doesn't mean I won't be your shadow and know your air supply during the entire dive. I almost always turn the dive based on my buddy's air supply before they do. That's my job and I take it seriously.