I'm not aware of any statute or precedent (and you've suggested none) that would prevent the deceased's attorneys from arguing there was a duty of care arising from the relationship between the parties, and at first blush, it seems to me (and my imaginary jury) to be a credible argument.
It was a commercial dive therefore there was an employer, and in this instance a contractor and the dive site was a workplace. I think you'll find under the relevant legislation: OSH Act of 1970 | Occupational Safety and Health Administration the Company who engaged Sotis is the employer and Sotis Company was that of a Contractor.