We need to separate criminal and civil (tort) requirements.
For a non-commercial trip, to run afoul of criminal law, a passenger would have to be injured through your grossly negligent or reckless actions. The exact wording depends on your own state laws. This would be hard to prove and most jurisdictions won't even try these sort of cases for recreational activities unless it's blatantly obvious. It would normally take something like abandoning the diver in the water, driving over a diver while drunk, or taking the boat out with known and dangerous deficiencies that directly cause the injury.
I suspect you are more interested in "how do I keep from getting sued if something goes wrong?" And the answer is that you can't keep from getting sued which is why you want liability insurance if you take anyone in or on your car/boat/plane/motorcycle/horse/pool/trampoline/?. That said, to win the suit, the basic question will be whether you acted reasonably based on what an average person would do in the same circumstances. For a boat owner, having a first aid kit on board is probably reasonable, and certainly if it is required by your local laws. If you have one, making it available to the injured person is reasonable. What they decide to do with it is not your responsibility. OTOH, emergency O2 is not something that an average boat owner would necessarily have on hand for a recreational dive. However for a technical dive, even a simple deco dive, I suspect a jury could be persuaded that not having it was unreasonable.
The bottom line is taking ordinary care is your best defense. Despite public perception to the contrary, US courts and juries are very reluctant to give out damage awards to people voluntarily participating in sports considered risky such as diving.
By the way, a properly written liability waiver won't keep you from getting sued. But it could absolutely help you get a speedy dismissal of the case as long as you didn't act with gross negligence.
