Agreed, but here in the US (where we have to be concerned with lawsuits coming from every direction) we were required by our insurance company to have “a person of supervisory certification on the vessel to oversee all diving”. On an OC punter boat that’s a DM. On a tech boat, that’s a Tech DM minimum. As stated upthread, I hired a mod 3 rebreather instructor (for dives planned to 100m or shallower) as the vessel Diving Safety Officer, not to regulate the diving, but to tell me when someone planned something stupid. If we were planning dives in excess of 100m (our deepest sanctioned diving was 166m) we had an expedition instructor. That’s an IANTD cert. Again, not to regulate the diving, but to guide and oversee gas planning, support divers, etc.
Mostly in the name of satisfying the insurance company, and to let me know if someone was either planning or doing something stupid.