1. I agree with Don: Watson was never much of a diver. However, the fact of the matter is that most divers are not particularly good. I recall seeing a statistic to the effect that most people who complete OW dive once or twice after that and then never dive again. Of the rest, most dive once or twice a year while on vacation. Only a small fraction of those who complete OW do AOW or rescue. And, even of those who do, my guess is that most still do not dive regularly or practice their skills on a consistent basis. I could well imagine any number of divers thinking they were good without being any good. I could also imagine any number of divers coming to the same, probably incorrect, conclusions that Watson seems to have done. But, being a bad diver does not equate with murder.
2. I, too, would be interested in info from Tinas computer.
3. It is not Watson's obligation to present evidence about his prior dives. He would not even need to say he did safety stops on every dive. An expert could present evidence that good practice includes a safety stop. Then it would fall to the prosecution to seek out his prior dive buddies in order to show that he did not always do safety stops. If the prosecutor does not do this, a jury might wonder why and might conclude that Watson's slow ascent was proper.
4. There is lots to the case that does not pass the smell test. This includes both Watson's explanations and key evidence that just seems to having been "discovered."
5. Where this discussion will be most useful in the real world is in helping the prosecution and defense get a sense of how real world jurors might evaluate evidence and arguments. I have had cases in which we have had professionals conduct focus groups to help us understand how normal people might view evidence or arguments. We, here on SB, are not as normal as those who would be used in a focus group, but given the costs of conducting focus groups, we might provide some good preliminary guidance. (Recall that a guilty verdict requires a unanimous verdict.)
6. As to the golf case that Don mentioned, the basic rule in most jurisdictions is that there is no liability for an injury that results from a risk that is inherent in a particular activity. Many courts talk in terms of it being contrary to public policy for the courts to change the rules of the game by imposing duties on the participants. One example is in baseball, where a bean ball is contrary to the rules, but is a part of the game; a pitcher is not liable if he hits the batter. Or, a late hit or clipping or a facemask in football may result in a penalty, but does not result in liability. However, there IS a duty not to increase the inherent risks, such as by hitting a ball without looking at where other players are.