I don't know rules, but I wouldn't want to be diving on this charter. Is the DM or other tending the boat fully familiar with how to maneuver the boat, use the radio to call in an emergency, give Oxygen, respond to other emergencies, etc? In most cases, unless the DM is also a Captain, I doubt it.
What if 5 minutes into the dive one buddy group surfaces and one of the divers has just had a heart attack? After getting his buddy out without any help from the boat, then they waste 30 minutes of valuable time because they can't figure out the marine radio to get help.
What about the anchor coming loose (something I've seen happen), what about if divers surface in strong surface current away from the boat?
Overall unless it is a site 5 meters from shore on a small lake, I wouldn't want to go on any charter without a Captain aboard the whole time.