As my husband would say, leave questions of law to the lawyers. Liability will depend on the law of the jurisdiction where the issue occurred.
Can they be sued? In the US, absolutely; you can sue for anything. Will they win? If they can show that the procedures followed were the standard of care for similar operations, the dive op may prevail.
Think about it . . . I've done dives where the briefing said, "Don't go here or the current will sweep you to Japan." If I go there and get swept, is the boat at fault? If I have a heart attack on the bottom and never come up, is the boat at fault? If my reg breathes wet and I panic and bolt for the surface and embolize, and sink before anybody sees me, is the boat at fault? There are a whole host of reasons why a diver might be missing, many of which involve no wrongdoing on the part of the dive operator at all. Whether the law or a court will see it that way, however, depends on where you are, and sometimes on the luck of the draw.
