Vernacular varies somewhat organization to organization, but Roel is correct in his statements. There is a difference (confusing I realize to many people) between "Dive Master" and "Master Diver"
(again, the below definition varies somewhat based on the certification organization)
Dive Master - professional level diver trained and certified to guide other certified divers on dive tours and to assist with teaching scuba certifications while supervised be a full instructor. Must have current liability insurance for the above activities.
Master Diver - non-professional level of recreational diver. Usually takes a minimum of 50-100 dives, along with various prerequisite courses such as advanced and/or rescue diver courses.
All that aside, I know many "professional" dive masters who haven't made a dime either...
As to Roel's questions, I'll take a stab at it, and there are plenty of others to suggest or comment here.
Dive Sites - Blue Hole you mentioned, I was also fond of Half Moon Caye Wall, and heard Elbow if good. As for closer in sites, they all vary somewhat site to site, but quality is in the same general category of the sites I've had experience with. Lots of Spur and Groove coral formations, a few swim throughs, great diving on the whole.
Ambergris Divers - I haven't dove with this organization, so don't have any direct feedback. I haven't heard any negative reports, which usually surface more quickly than positive ones.
BBQ at BC's - I'll let a current local chime in on this activity.
((one of these days I'll learn to spellcheck BEFORE I post, especially early am Sunday))