I have not DMd any charters here because I am not familiar enough with most of the sites/wrecks. If I never do that I guess there's no need.
Dunno. I would think a "local DM" should be familiar enough with "local diving" to be able to comment on it. If you don't want to lead dives to these wrecks/sites, that's cool. But if a student asked a local DM about local diving and the DM said "Um, yeah. I'm really not familiar enough..."
Get out there, man!
:d