And I'm telling you the untimate responsibility is the boat Captain's alone. No doubt the DM was a major contributing factor (along with the knucklehead that answered for the diver a roll call), but in the end the Captain sets and enforces procedures on the boat...........Then they are clueless about how ruthless the sea is in punishing the unprepared. The rules that hold the Captain ultimately reponsible have been developed over centuries of experience by professional seamen, with the rules written in the blood of many dead men. Through my long personal association with the San Diego long range sportfishing fleet I have seen time and again the sea's unrelenting desire to kill you.........If this Captain had established proper procedures and enforced them then there would be multiple levels of checks such as verifying not only a "here", but making visual contact with the responder, then checking for the number of tanks with regs/BCs attatched, then maybe move to the next site.
Probably good this was a career ender for the DM, she sounds unsuitable for the job, but the Captain needs to grow some balls and take responsibility for his failings in this incident, then move on and be a better, wiser Captain for the experience.