This is an interesting and perhaps not totally resolved issue.
To qualify for negligence litigation, a physician and patient have to enter into a bona fide physician/patient relationship. This is not a simple thing to define. For example, if I am on call for a given specialty for an HMO, and a patient calls me, or even simply books an appointment with my office without speaking to me at all, for an issue in my specialty, a relationship may be established without personal contact or examination.
On the other hand, I can examine a person for hours for the purposes of an independent insurance exam (say, for workers' comp), yet the law recognizes that such an exam does NOT constitute a relationship for malpractice purposes.
Whether a relationship can be established online only is a matter of some controversy, as many companies dispense drugs like viagra online based on the notion that such a relationship can indeed be set up without any personal contact. In this case, however, the websites are setting themselves out as medical providers, supplying the names and credentials of the doctors, prescribing drugs, taking histories and charging fees.
The facts that 1) no scubaboard MD is obligated to treat a poster, 2) the board does not advertise itself as a provider of medical services, 3) the postings on both sides are basically anonymous, or can be, so no rational person would reasonably expect to establish a relationship with an anonymous provider of unknown credentials; 4) there is no compensation for services; 5) there is no physical contact...all make of these facts render it virtually impossible to argue that a true provider/patient relationship could be established in this forum, making negligence a non-issue IMHO
Actually, this may be a better questions for the lawyers...