If you're going to be on the island for 4 days as mentioned before I'd sure consider doing 2-3 days at the Casino. Plenty to look at and explore there. Then hook up with one of the dive boats on the island (since you're already there) and dive a bit more.
I just did 2 days off Casino and could spend a lot more time there very happily.
When diving at the Casino the Catalina Divers Supply has a trailer with tanks, weight, and lots of other rental stuff right next to the casino building. Pop out of the water, trade tank, relax a bit, and right back in. I think the tank and first fill is around $16 and $7 fills.
Look for the plaque dedicated to Jacques Cousteau. It's about 55 degrees from the steps in about 40' of water. Pretty cool. I haven't seen them but there's a couple of 'wrecks' to be seen there. There are also a couple old marine engine blocks that are neatly camouflaged to look exactly like rocks. ~30feet and maybe an 1/8s of a turn left off the steps. Easier to spot spot all the cables that drape them. Kelp right now is pretty low due to the heat, but in November I'd guess it should be better.