I'm there all the time (in fact, yesterday... )
The answers is: depends. Some options to consider:
Stay on the mainland #1: Come down to So Cal, and grab a local dive boat. They typically do 3 dives in a day at various spots around the island. Cost for these board is about $90 - 129, depending on the operator.
Stay on the mainland #2: Come down to So Cal, and grab the Catalina express to the island, and dive in the dive park. Cost for the Catalina Express is $43. Departs from Long Beach, San Pedro and even Dana Point (I think that one is a little more expensive). Its about an hour to make the crossing, and diving in the park is free. There is a place for air fills ($5) and some and two shops on the island you can rent gear from.
Stay on Mainland #3: Come down to So Cal, grab the early Catalina Express and once there, get on board one of the Catalina based dive boats to dive various spots on the island. This is not recommended - as you pay for the express to get there ($43) and then pay for the Dive boat (another $85 or so)... its silly, but people do it. Take a So Cal based dive boat. UNLESS, you're staying on the island (see below...)
Stay on Island: You can dive the park for free. You can grab an Avalon-based diveboat (I believe there are essentially two) or you can rent a boat and go exploring.
If you want additional info, let me know and I can move this thread to the So Cal room where you'll likely get more lovin from the locals who dive there all the time.
K