I have done some brief research on this idea that divers have been suing each other for being bad buddies. I searched all state and federal cases in Michigan. I found no reported cases remotely similar to this scenario. I haven't searched outside of Michigan, so maybe there are some elsewhere.
One of the posts above links to a Rodale's site, but there is nothing on the site about this specific issue. Some posters on this thread have said they have heard about such lawsuits. Is this something you have actual knowledge about or is this just another "scuba legend" ? If there has ever been such a suit, I'd really like to hear about it.
The general rule of law is that absent special circumstances you have no duty to rescue somebody in danger, even if you could do so in perfect safety. If you do attempt a rescue, the common law requires that you do so without acting negligently. So some people felt they were better off doing nothing rather than attempting a rescue. As a consequence, some states, like Michigan, have "Good Samaritan" statutes, which exempt a rescuer from any liability except for intentional injuries or gross negligence.
How this shakes out in practice is heavily dependent on the facts of the situation. Examples: If you are responsible for a diver, such as if you are his or her instructor, you have a duty to perform your job competently. You are liable if you don't and that failure causes harm to your diver. If you are just diving in the same ocean, and you happen to be nearby when some diver you never saw before goes out of air, you don't have to do a thing. Legally, not morally. Diving with a buddy is somewhere in between these points. If you buddy up with somebody, you're undertaking to be a buddy and not just take off when you get under water. You have a certain obligation, i.e. a duty, to do for your buddy what a reasonable buddy would do. You have to act as a reasonable buddy would act under the circumstances. If you don't, you could be liable.
This is the same standard you are held to in ordinary dry-land life. You have to act reasonably. If you have a duty to others, you can be liable if you don't act reasonably to discharge your duties.
