My advice is to not sign up for a dive with the cruise ship. Instead, go to Catalina Diver's Supply directly to rent gear, then take it over to the Casino area (big round building -clearly visible landmark) and dive in the park there for free. Of course, that depends on what they're charging you, but the diving at the Casino is free, I would imagine that rental gear wouldn't cost more than $60 each for all day use, plus another $5 per fill, depending on how many dives you want to do. The dive park is a gorgeous place, too - lots of everything, kelp, some wrecks, shallow reefs, deep reefs, etc..... Plenty to keep you busy for at least 3 dives (heck I've done 30 dives there and haven't seen it all!).
Now, if they're charging less than $100 each, it might be worth it to go on the cruise outing and have a guide/everything arranged, etc.... But, for more than that, which I think cruises usually charge, I'd just do it on my own.
If you do choose that, it's pretty easy - you'll see the steps in the rocks to enter the water - just watch the rocks the first few feet out of the steps and you're all set. Good luck!