Now to address some of the other posts ...
Is there anyone on the boards here who will say that there is no possibility that Tina's death was an accident and that Gabe's bizarre conduct afterwards was just a manifestation of a combination of bad manners, bad judgment and grief, amplified by peoples' desires to blame him for Tina's death?
To me, the single most damning evidence against Gabe is the slow ascent. I consider myself a decent diver. I think I can follow my training. Even so, if my new bride, who took up scuba diving at my behest, got in trouble on a dive and I needed to get help, I expect I would handle it as a CESA rather than as Gabe seems to have done. Under the circumstances, the slow ascent gives me a lot of trouble.
A comment about what Gabe allegedly said at the mortuary ("I'm so sorry, I never meant to hurt you. I shouldn't have kept taking you down. I'm sorry, I couldn't stop.''): Is this wholly inconsistent with what a grieving husband might say if his new bride drowned after taking up diving so she could enjoy diving with him? Should he have had the presence of mind and been articulate enough to say: "I'm so sorry I encouraged you to dive. I never meant for you to take up a sport where you could get hurt. I should never have taken you diving with me. I so enjoy diving and enjoyed having you enjoy it with me that I just could not bring myself to tell you to stop diving."? Is the latter a lot different from the former?
As far as the card he purportedly sent to Tina's friend: The wording was probably tacky to most readers. But, maybe it was a poor attempt at opening a dialog. Just think of all of the tacky "pick-up" lines one hears at bars or other social gatherings. Some people can't come up with anything better.
As far as whether Gabe was "hitting" on Tina's friend or looked for subsequent relationships, if he was truly fond of Tina and of being married to her and her death was an unfortunate accident, is it utterly inconceivable that he might want to replace that feeling as quickly as possible? Before answering, consider how many people jump into new relationships as soon as they break up from an old one. Why do they do this? Answer: Because they want the good feelings that supposedly come from being in a relationship. Why shouldn't Gabe have sought to replace the feelings if he truly loved Tina, etc. If he was unhappy being in a relationship with her, I could see him avoiding another relationship, not trying to get into another one.
Finally (for now): In a criminal trial, the defense generally does not need to announce its theory of the case until after the prosecution has completed the presentation of its case. As a result, the defense can adjust its theory to account for what it learns during the prosecution's presentation of its case. Thus, depending on what the prosecution presents and how the defense perceives the jury's reaction to the prosecution's evidence, the defense can decide whether to frame its case as "Scuba diving can be dangerous and Tina's death was just an unfortunate accident;" or "Gabe and Tina planned on scamming their insurance company and their plan went awry." (Note: The second may expose Gabe to jail time, but not necessarily for murder.)