Good question, in NJ the DM usually ties on to the wreck, then comes up and does the brief, seen the same in NC. One exception is where a group/club/shop charters the whole boat and brings it’s own DM’s
In Florida the DM goes on the dive, not always leading but then I’ve seen mutliple DM’s on Florida boats too plus photo guides. Been out on JA’s boat out of WPB where it seems every photographer has his own DM/guide. Same is true for most tropical travel spots and places like Coz where the live boat is following the DM.