All my Catalina Dives have been from my own boat, so I'm no expert on the dive boats there, but here goes:
Check out the Catalina sticky thread at the top of the SoCal forum. The 5th post has information on the two dive services that operate out of Avalon. Catalina Divers Supply's boat is smaller than Scuba Luv's, but both have received good reviews from those who have used them. Some of the Best diving in Catalina is at the Istmus (Two Harbors). If you can't get a dive trip there from Avalon try the Two Harbors dive shop boat:
Scuba Diving - Visit Two Harbors, Catalina Island
You can take a ferry or a bus from Avalon to Two Harbors, although schlepping equipment on the bus could get old quick. Both trips are worth it for the sightseeing alone, from the boat you will see about half of the southern side of the island and from the bus you will see the interior, and if your lucky you might see some Buffalo. There is a small Hotel with a good Restaurant at the Isthmus, the Banning House Lodge, worth a night if you can get a room.
Ask at the Harbor Office about snorkeling with the leopard sharks. Awesome experience!!!
Make sure you try some of the local white sea bass at the restaurants. They were almost fished to extinction, but a farming operation by Hubbs Research, with a lot of volunteer help, has brought them back to commercial levels. Delicious.
And speaking of Bass, when you dive from Avalon, see if they know where the Giant Black Sea Bass are hanging out. I've only sen them once, and ranks as one of my favorite dive memories.