ItsBruce - interesting idea, however, I always wanted to see this case go to trial, no matter what the outcome. I am hoping that through the process of a trial, we will learn more about this case. We never got that chance with the plea agreement.
I think a couple of theories about the "turning off of the air" was floated around and in the media there was an assumption that the bear hug was Watson turning off her air because it was more expedient to describe it that way. In that case, Watson would not have had a chance to turn the air back on (and her air was on when she was found). However, the bear hug could also be interpreted as Watson turning the air back on after having turned it off. The witness did not see Watson's hand on the tank valve as the witness was focused on the fear in Tina's face as she was in the grasp of Watson. The importance of the "bear hug" if you accept the witness testimony, is that it conflicts with Watson's statement that he could not reach her and he was trying to reach her and was swimming down furiously after her. The diver who had Tina in a bear hug, according to the witness, let go of her, let her float to the bottom and swam away from her. You could draw the conclusion that diver was Watson since no one else was with Tina and Watson's statement would also corroborate that no one else was in the vicinity as Tina sank (at a high rate of speed) away from him to the bottom. In addition, the prosecution claimed that Watson's computer did not show a sharp descent to swim after Tina as he had claimed. Watson's computer corroborates the witness statement. So you have two strikes against Watson's statement - the witness and his computer.
At this point, you would have to analyze the police interpretation of the computer data. I suspect this could be an issue at trial. In other words - on Watson's computer, how far down could you go and come back up without the computer registering in what time frame? I did find information about Watson's computer and gave a link to the computer manual in the "Issues" thread. So perhaps the defense could make an argument that the prosecution did not interpret the computer data properly, but then they still have to contend with the witness testimony. In light of the witness testimony, the jury could potentially give more weight to the prosecution's interpretation of the computer data.
I would also say that the witness testimony of seeing Watson have Tina in a bear hug and then letting her go to sink and swimming away from her could be more powerful than the argument that the witness did not see Watson's hand on the tank valve turning it on or off. What I mean by that is, one argument has much more powerful imagery for the mind of the juror than the other and has a greater chance of "sticking."