[QUOTE=tugsailor;162988]I think the 12 passenger rule comes from SOLAS, and its implementing regulations. Under SOLAS, a ship inspected as a freighter may carry up to 12 passengers, but a ship that carries over 12 passengers must be inspected as a passenger ship. I do not know what triggers a requirement for a doctor on a passenger ship.[/QUOTE]
There is no trigger, at least not under US law, as far as I can tell. I believe SOLAS does not apply here, either. Cruise ships carry a doctor for practical and competitive reasons. Since passengers are not required to pass a physical, it makes sense.
However, there may be applicable requirements from other flags of registry.