quietstorm: when was it that you dove the St. Anthony?
It's interesting that there was no life to speak of -- either this was before it became populated (as in, the artificial reef was just created) or the sea life had good reason to not be around. Did you happen to notice any shadows out in the blue, or feel like you were being watched? Tiger sharks have been known to cruise in and around that area (not usually seen, but I've seen reefs go completely void of life just about instantaneously as the fish all find their hiding spots).
As for second dives -- though few operators do this, I enjoy doing both dives at Molokini... there's enough to explore there that you'll be hard-pressed to use up all the sights in even a dozen or so dives.
With regards to the Backwall.
As far as I know there is really only one operator that will "guarantee" a Backwall dive, and that is Maui Dive Shop that does it as a 1-tank excursion some mornings. Then they return to the dock, swap out passengers and go back for a 2-tank dive in the crater or along the coastline.
Because of this out-and-back business, and that the Backwall's conditions are never predictable, I really can't recommend this option.
Other operators that are likely to do the Backwall: Mike Severn's Diving and Ed Robinson's Diving Adventures (I also cannot recommend the latter -- PM for details). Of course, this all depends on the experience level of the divers on board, as it will with both Prodiver and Makena Coast Charters that happen to do this dive every so often as well.
