I can give a firsthand experience concerning this;
Last year I was chartered for 2 days by a private survey company doing bottom surveys for the Corps of Engineers. They wanted to carry 8 pax, one of which was a COE safety observer, as required by regulations, I explained it could only be 6 total to meet the boat's restrictions(I personally have a 200T(domestic),500T(ITC) Masters license. They complied with that and the first day's trip was run. Upon returning to the dock, I was met by a USCG MSO and NC Marine Patrol who explained that they had received a report that I was carrying excess passengers(I later found out another charter company had called in the complaint as harassment). I had the 6 pax, myself and my mate onboard for a total of 8. My safety equipment was checked and the boat's drug program paperwork that is required to be kept onboard along with my license. I was in compliance with regulations so no citations were issued.
Based on my conversations with MSO's here in NC and research from this and a previous incident, this is how the CG here interpets the regs; onboard personnel are classified as either passengers(there is no distinction between paying and non-paying) and crew(proof that someone is a crewmember is enrollment in the boat's drug program). There is not a crewmember limitation on Uninspected vessels other than the vessels weight/balance restrictions.
When I asked about the divemaster/intructor situation, the CG does not allow any exceptions to exceeding 6 passengers.
I understand there is differing enforcement/interpretation of the regs between CG districts, but the maximum fine for exceeding passenger limits is around $25,000, so I'd be real clear as to how it's viewed in your district