I've dived Catalina Island from a Day Boat out of Long Beach, I've dived it on Day Boats out of Avalon while staying at a hotel there, and I'm going next month to do a 3-day Liveaboard to the Channel Islands, so - let me know if I can help with this...
Having first dived the Caribbean and Atlantic - from West Indies to Florida to Roatan to North Carolina - going to the Pacific was a
big change! The reefs are rock rather than hard corals, but the Kelp Beds and other cooler water algaes are mind boggling to dive. Many totally different fishes and invertebrates. I know that many east coast divers never try the other coast, thinking they have too much to see for one lifetime - and while I can appreciate the truth to the thought, the variety is certainly worth the trip. Besides - the flights, hotels, dive bags and all are part of the adventure. While not as convenient as diving one's backyard, the challenges have their rewards.
The waters can certainly be
20 degrees cooler off the west coast as off the east, but - I have a source for 7 mil suits for $120. Neoprene beanie, gloves and boots is about all most need, but it also helps to pour warm water down your back before each dive.
Avalon is a darling little seaside town, with something for everyone. The tour to the airport is a hoot, and I especially like the succulent garden. Never seen so many blooming Century Plants!
See:
http://www.catalina.com/main.html