There are a few ways to dive at Catalina:
1. Get on a dive boat from mainland and go for it. Advantages are that the boat will go to several different sites, and you can find an insta-buddy with ease.
2. Get on the Catalina Express and get out to Avalon (the city on Catalina) and shore dive at the Casino Point dive park.
3. Get on the Cat Express, get to Avalon and get on one of the dive boats being ran by two main Avalon dive shops (Catalina Diver Supply and Scuba Luv). To me, this is a bit superfluous because you have to pay for transit on the Cat Express and then pay another dive boat to take you around the island. The only way this course of action would make sense is that you were to do your vacation on Catalina itself and wanted to dive at places other than the Casino Point dive park. Otherwise, it's cheaper to get on a dive boat from mainland.
Anyway, if you're at Catalina for the whole weekend, then maybe it wouldn't be bad to do Item #3. Dive at Casino Point on one day and the next day dive with a boat chartered through either Catalina Diver Supply or Scuba Luv. From what I understand, both of these boats have excellent services. Of course, they can provide you with all the equipment as well.