Keep it simple, go to Catalina. Some of the best and easy entry diving right on the Casino Point and two dedicated dive boats ready to go for more variety. Besides, its a world apart from California the way most think of. Lots of other land based stuff also. No need for car rental.
Diving the Dive Park is great but it is a sliver of what is available. Yes, they have a couple boats that leave from there and dive the rest of the island, but the cost of the ferry to get there and the dive boat costs is more than a mainland based boat trip. Add a room cost for multi day diving and it gets expensive fast. Plus, the ecosystems at the other islands are varied and different from Catalina. I'll dive Catalina any time (and have over 300 times) but really enjoy the trips to the other islands more and especially for multiple days.