Boats like Blue XTSea's , Scuba with Tony, Challange Diving, all but one of Aldora's - boats with smaller amounts of permits - the ones referred to six paks - those mostly run with a captain and DM, sometimes two DM's.
Boats often referred to cattle boats will have a separate crew - Dive Paradise, Cozumel Marine World, Scuba Du - and a lot of times multiple DM's
There has to be a maritime rule or something covering that, there's rules for everything.
I've been told that you have to be a natural born citizen here to be a captain and I don't think non nationals can get their seaman card, maybe once you're a citizen but still unsure. The work around the seaman card is international ones are recognized, so I'm told.......
The seaman card / work permit - I've dove a lot with one shop taking classes, DM, Instructor and specialties - we technically aren't allowed to handle lines while docking even though it's part of the divemaster class. You'll "lead" dives, "teach" classes but all under the eyes of someone who is really allowed to do it, that would be working and you aren't allowed to do it - let alone not having the marine park card.
I've seen Instructors from the US completing classes or even teaching classes down here - I'm told it's a big no no - not worth risking getting deported over.