Hmmm... well, I'm not a lawyer, but...
One of the things that goes into figuring a reward in cases of negligence is the concept of duty. I know something about medical malpractice, so I'll use that as an example.
For a medical malpractice to exist, there needs to be three things: (1) a duty to care for the patient (i.e. a doctor-patient relationship), (2) neglect of that duty, and (3) damages.
So if I write an article that says that drug X is good for condition Y, and someone with Y takes that drug and has a bad outcome, I'm off the hook as far as medical malpractice, since I have no duty to care for that patient (3, but no 1). Similarly, if a patient comes to my office and I give him a drug but forget to check his chart to figure out that he is allergic to that drug, and he takes it and does not have an allergic reaction, I'm also off the hook (1 and 2 but no 3).
SO, the reason why the DM's negligence might be considered relevant and Dan's is less so, is that the DM was acting as a professional to provide a service, and the jury concluded that accurately checking people in and out of the water was part of that service - that the DM had a duty that was neglected.
To extend the analogy with medicine - I see a patient and do a biopsy. The results come back as cancer, but the pathology report gets lost on my desk and I never call them. But the patient also misses a follow up visit, and they never call me to get the results.
If they had come in for a visit or called, they would have gotten the results, so their actions were definitely contributory to missing the diagnosis. However, that means nothing, since the duty to care is mine, and malpractice would exist.
Maybe the lawyers here can discuss the differences between the medical and dive industries in this regard - I'm sure that there are some. But I think that the similarity does exist in the question of the duty of the professionals involved in this case. To say that the award should be nothing because if Dan hadn't drifted off, he wouldn't have had the problem, does not acknowledge the role of professional negligence, IMHO.