Caveat, I have done two boat dives this past June. _Very_ limited experience.
What I saw was that the mooring balls for the common dive sites on the West side hide about 15 feet underwater. The dive boat pulls up to the location that they know has mooring points. The DM is on the bow, the captain idles around to find the actual mooring ball, and when they get close enough the DM dives in with a line and connects the boat to the ball. I don't know if this is to preserve the aesthetics of the sites or to try and keep privateers off mooring balls set by commercial dive groups, or maybe simply trying to keep mooring balls from being navigational hazards? I dunno.