I think that my various posts have explained why I do not see any legal liability here. To summarize, for there to be legal liability, certain elements must all be proved. One is that there is a duty. Another is that a breach of that duty was a substantial factor in causing the injury. If one or the other is not proved, there is no legal liability.
I do not see a court finding a duty to either prevent the particular dive or to rescue a distressed diver, especially if the distressed diver has not surfaced. I do, however, see a court finding a duty not to leave a diver behind.
Assuming the court says there is a duty to have prevented the diver from diving without a buddy, I have a hard time believing it would even let a jury consider if that breach was a substantial factor in causing the decedent's death. An expert on diving would have to testify that to a scientific certainty, the presence of a buddy would have prevented the diver's death. Anything less than that, e.g. it might have prevented the dive's death, would not even be admissible in evidence.
Assuming the court says there is a duty to rescue the diver, one would have to prove that he could have been rescued. One would need evidence that the dive had not died prior to the time that (1) someone should reasonably have figured out that he was in trouble, (2) someone could have located him, and (3) someone could have rescued him. That is very tough, but maybe his dive computer might help.
As far as whether a breach of the duty not to leave a diver behind results in liability, one would need to prove the diver was alive at the point the boat left, Otherwise, all you have is the boat leaving a dead body behind. Terrible though that is, it is not a basis for liability.
Now, for anyone who says the dive boat / operator / DM is liable, please provide a reasoned analysis. There is no need to say that leaving a diver behind is inexcusable. I'm pretty much in agreement there. However, excusable or not, that does not satisfy all of the elements necessary for liability.