My husband and I were just there in Dec 06 and dove for two days with Aqua Adventures. We booked with them even though they were slightly more expensive than other shops because the person working the desk told us that they were only one of a very few dive shops registered with PADI. I didn't check up on that, but thought I would mention it. The first day of diving was outstanding and the service from the shop was good. The second day was good diving (I saw a turtle!), but they sent us out with a young guy as dive master who had NEVER dived the site before that day!!! They apparently hired him off the street the day before. That was very unimpressive I thought, and I believe the guy drifted all of us way off the reef because he didn't really know where he was going. The group sizes and equipment were good.
By the way, there is fun snorkeling right around the Avalon. There is a site on the back side you will find, but the front right side by the rocks is great!
There was a guy on the road to town selling popcorn that we bought every night. It's a great town - enjoy!