Mark, check manning requirements under 15.805 and you'll see this:
There must be an individual holding an appropriate license as or a valid MMC with endorsement as master master in command...
As a 100 ton master myself who has worked on dive boats I know it's common in some smaller operations for the captain to also be the dm but it's really not well advised. In an emergency
situation you might need to be providing first aid at the same time the vessel is needing to hightail it back to shore.
Divemasters can and do get injured themselves chasing after bad divers. A guy on a local diveboat here in San Diego
said that he gets in the water almost every day because he has to help someone who really isn't fit to be in the water.
I don't think you'll get a lot of leeway out of the USCG if they happen upon the boat and they ask the crewman onboard where the master is and he says he's blowing bubbles at sixty feet.
If you're a licensed master it shouldn't be too hard to find a divemaster (or send your crewmember to padi divemaster classes). Another battle you get to fight is "who's responsible for the dive operations?" Some boats say it's the captain, some say it's the divemaster. The natural "the captain is responsible for everything" doesn't work all the time because it's not like the captain of a cruise ship
is responsible for the casino onboard. Some operations onboard really have nothing to do with the operation of the vessel and safety
of the passengers from a vessel point of view. People 100' below the surface kicking up silt in a wreck really have no connection to the vessel any longer (and probably never will again).
Anyway, let me know what you do. It's an interesting area of the water but like I said the USCG probably isn't going to like seeing passengers for hire onboard a boat without a master. What if you surface with some injured divers a few hundred yards away and need the boat to get over to you?
If you're the master, stay with the boat. Split the load with the dm.