One more thought: If David killed Shelly and did so according to the prosecutor's timeline, what did he do after that before surfacing? He could have done a tour of the wrecks so he'd have pictures to support an alibi. So, why didn't he use some of his time to replace the fin, put the regulator back in Shelly's mouth, find the mouthpiece of the snorkel or dispose of the snorkel entirely? In fact, if David did it, he would have been better off to say he saw Shelly having a panic attack and that he tried to help her but failed.
Let's assume for a moment that David DID do it.
Bruce, you seem to be assuming that the fin came off in the struggle, the mask broke in the struggle, and the snorkel came off in the struggle. And you wonder, rightly so if that were all true, why David simply didn't "fix" all of that?
Well, let's also assume for a moment that NONE of that happened. The mask was on, the snorkel was intact, the fin was on her foot.
Now, David's going to try to make it look like something terrible happened to Shelley to divert attention from him. He's already a little panicky in that he just killed someone and he doesn't want it to look like murder. So he "stages" the scene. He simply doesn't realize that he's not doing a very good job of staging and will actually cause more questions to be asked that don't have logical/good alternate explanations.
David removes the fin and sticks it in the sand. He rips the mask off her face and the pin comes out. He pulls off the snorkel and yanks off the mouthpiece. The reg's already out of her mouth so he leaves it out. And maybe he even moves the body closer to the wreck so it looks like she could have bumped the wreck and panicked. (Remember, we only know for certain where Thwaites says he found Shelley. That's not necessarily where she passed out, regardless of the reason.)
Then David swims off to the other side of the reef to eat up some time, gives himself an alibi, and make sure Shelley's dead. Then he surfaces. (I have some other questions about the way he describes his dive but I'll deal with those in a separate post.) David's fairly confident that Thwaites will not have gone down until David gets back so he casually asks if anyone's seen Shelley yet, and waits while Thwaites begins his dive, only to find Shelley on the bottom.
You're right in that David should have said he saw Shelley having a panic attack, tried to help but couldn't. But - again assuming he killed her - maybe he's not as clever as he thinks he is.
I'm not saying that this is the way it happened. But I am saying that this is one of the possible scenarios you have to consider when you look at the totality of the evidence.
- Ken