Avalon is easily accessible by air (LAX, SNA, or LGB), or by ferry from Newport Beach, Dana Point, or Long Beach. There are three dive shops within walking distance of the ferry and the casino, a prominent landmark, but the most conveniently located is Catalina Scuba Luv, a green structure right on the pier. I take the ferry from the Balboa Pier (Newport) and travel lightly. I brought my own fins, mask, regulator, BCD, and other accessories because they fit in one seabag (relatively portable), but I rented two air cylinders, weights, and a cart to transport them. Water temp was 58-60 (F), and I wore a 6.5mm wetsuit. Lodging expenses vary from the very affordable to the extremely well appointed and expensive, but everything in Avalon is within walking distance.
Shore entry and egress at Casino Point are simplified by concrete stairs and handrails built in 1997. The visibility was 35 ft+, the marine environment was healthy, and biodiversity reflected a good spectrum of the marine life one would expect in this area, e.g., garibaldi, California sheepshead, bat rays, lobster, blacksmith, calico bass, and others. Dive the same site a few months later and you'll see other creatures. Anyone who dives the casino and is not pleased with the experience has some other limitation beside poor timing. Marine life is abundant. If you're comfortable working in kelp, you will discover gems (like the blue-banded gobies) at the holdfasts.
Quick Tips:
If your priority is a diver's getaway (i.e., eat, sleep, dive), you will not be disappointed with the convenience and amenities Avalon has to offer. If you're on a very tight budget, you can stretch your food budget. If you visit the island as a couple, there are romantic dinners and fine wine choices in several excellent restaurants nearby. It depends on how deep your pocket may be. I encourage you to accept the challenge of leaving your cell phone behind.
Best Way To Get Around:
All roads out of Avalon rise to higher elevations, so buses will carry you to the airport or the Isthmus. Golf carts are available for rent. The ferries do not carry vehicles, but you can bring a bicycle. I doubt you'll need it, since it's a pleasure to walk around and enjoy the slower pace.