Well, Roan and Frank O'Donnell know Catalina a lot better than I do, but I'll take a stab anyway.
I went there for a day on my birthday in 2003 w/ Roan. We took the ferry over. Not bad, but I'd recommend that you make plans to stay overnight if you can. I believe that you can camp on the island in certain spots, but I don't have my notes handy.
On all of the parts of the island that we dove, w/ the exception of Casino Point, the depths increased very quickly as you moved off shore. The south-east part of the island seemed to have more gradual drop-offs than the other spots we visited. Still, I think that the Casino Point scuba park might exceed 60ft out at the edges.
There is a fair amount of boat traffic, even on weekdays, so you have to be a little careful. Occasionally, you can find coves that have a nice combination of depth and treacherous rocks that keep the boats at bay.
We only went on the chanel-side of the island. I think the winds generally blow towards the main-land, so the channel-side generally has better vis and tamer waves. Vis was respectable, but not great the day we went -- maybe 30 ft. Roan said it was a bit below par for Catailna.
As you move from Avalon to Two Harbors, the underwater scenery changes pretty rapidly. We only went as far as White's Landing, I think -- maybe we actually sent past Long Point -- I can't remember for sure. It was 3 or 4 miles. Within that space, the underwater scenery changes quite a bit.
If you want kelp forests, you will find them. They are ubiquitous.
We used a kayak to get around. Seemed pretty effective. Watch the currents, though. We had to do a Hawaii 5-0 imitation, paddling the kayak agains a 1/2 knot-or-so current to get back to the ferry in time and we literally made it with only seconds to spare.
Good luck, I'm sure you'll enjoy it.