For those who are of the opinion that a dive boat is a "taxi service" with no responsibility for diver safety, it is possible that the dive operator could be determined to be at least partially at fault. Let me emphasize that I have no information other than what is in the thread now and have no reason to suspect that the operator's actions were anything but stellar.
Here is a summary of a thread a few years ago about a similar incident in which the coast guard severely punished the dive operation. My memory is a bit hazy, but the key details will be accurate. In the incident, a diver struggled on the surface, likely having a cardiac event, and died. The coast guard investigated and determined that the crew's rescue efforts were inadequate and that they had not been properly trained for such rescues. Furthermore, the captain was not on deck supervising at the time, as is required. The captain was strongly sanctioned for failing to supervise and for failing to provide adequate training.
Again, as far as I know, the crew in this incident was properly trained and supervised, and they did all that they possibly could to save this individual. If not, ....