I too have been considering a dive trip to La Paz and/or Loretto.
I checked out Club Cantamar and Cortez Club. Cortez Club sets up Package deals with La Concha Resort and a few others, I think.
I heard that Club Cantamar was pretty far from the town of La Paz, and that it was a fairly long and relatively expensive cab ride to get just about anywhere from their site.
On the positive note, I heard that Club Cantamar is right by the marina where to dive boats leave from.
Did anybody who stayed at Club Cantamar think it was too isolated? Other than the location, I've heard mainly good things about the place.