Some comments on Dadvocate's post:
1. In most jurisdictions, it is harder for the prosecution to convict someone in a criminal matter than for a plaintiff to prevail in a civil case. In a criminal prosecution, the prosecution must generally prove guilt beyond a reasonable doubt. In a civil case, the plaintiff must prove fault by just 50% plus 1. Thus, even if Gabe is not found guilty of a crime, he could be found liable in a civil case. (This is what happened to OJ, the first time around.)
2. The fact that diving can be dangerous is relevant only to the extent that it gives Gabe the opportunity to argue an alternative explanation for Tina's death, i.e. that he did not kill her.
3. In a homicide, there is a difference between murder, manslaughter and negligent homicide. Murder generally entails intent without justification, excuse or mitigation. If there is gross negligence, but no intent (and my recollection serves me correctly), the most one can have is manslaughter. There are generally two levels of manslaughter, voluntary and involuntary. The distinction is based on the existence of a justification or excuse or other mitigating factor. If there is only ordinary negligence (and my recollection serves me correctly, then the death is only a negligent homicide (and is rarely a crime).
My sense is that if one becomes separated from one's buddy as sometimes happens while diving, and the buddy dies, there probably is not even a manslaughter. If one abandons one's buddy and the buddy dies, there still is probably not even a manslaughter. If one abandons one's buddy while the buddy is in distress, there is probably manslaughter. If one abandons a buddy in distress because one is himself in grave peril or thinks getting help is a better idea, it is probably involuntary manslaughter. If one abandons a buddy in distress in order to continue with his dive, etc. it is probably voluntary manslaughter. If one turns off a buddy's air as part of a pre-arranged drill and the buddy dies, it is probably involuntary manslaughter. If one turns off a buddy's air as horseplay, it is probably voluntary manslaughter. If one turns off a buddy's air because one would rather have a different buddy next time ...
As a general rule, the law does not require one to place oneself in great peril to assist another especially if one did not put the other in peril. In fact, in most US jurisdictions, unless one either (1) creates the situation, (2) has voluntarily undertaken to protect another, or (3) has prevented someone else from acting, one can sit idly by and watch someone else struggle and die. In California, the law allows someone to sit idly by on the beach and watch another person drown, even if it would be easy to either help or call for help. There would be an exception if the one on the beach had obscured a warning sign or the one on the beach had assured other possible rescuers that the drowning was just an act as part of a class or the one on the beach had previously told the victim he would keep an eye on the victim and would summon help if necessary and the victim relied on that assurance when entering the water. Note, this is just some general principles and your mileage may vary.
4. As far as a civil suit in the US. I cannot see why that would not be allowed. As US residents, the US and the state in which Gabe and Tina lived would have jurisdiction over Gabe. And, the home state would also be deemed a proper venue. Whether the law of the state in which they lived or the law of Australia would apply is a bit more complicated. Nothing stops a state from applying some other jurisdiction's laws to a given case. It is frequently done and there is a lot of law on whose law applies to a given case. If he wanted to prevent the matter from being tried in the US, the best Gabe could argue is that it is an inconvenient forum because the witnesses and evidence are in Australia and cannot be brought to the US.
5. Unless there is a smoking gun, my guess is that the only reason the authorities are even looking seriously at the case is because of the inconsistencies in Gabe's story. But for those, the prosecution would have just about nothing. (Think of how many inexplicable diving deaths there are every year and how few prosecutions there are.) it is the inconsistencies that give the case teeth.
It is because of possible inconsistencies that many experts tell people to NEVER SPEAK TO THE AUTHORITIES. Consider the following: There is a bank robbery by an unknown person. The only evidence to connect you with it is that a witness comes forward and says he saw you two blocks from the bank. In this situation, the police have nothing. But, if you say you were somewhere else, even if true, the police have an inconsistency and that leads to a suspicion and further investigation. And, if you say anything more, there may be more inconsistencies. (Of course if you have sufficient, credible, independent witnesses who can unqualifiedly identify you as being elsewhere at the exact time, you may get off. However, you may get to spend a few days in jail and get to pay for a lawyer in the meanwhile.)
