I think Lee is being a little bit harsh. No, you won't have 100 foot visibility and the water is more likely to be green than blue, but you don't really NEED lots of viz as the stuff you're likely to see will be small and on the rock reef and kelp anyway. Since you'll need to rent gear anyway, just rent a 7mm suit when you pick up the rest of your gear. The islands generally have better visibility than the shore sites, so head out to Catalina on any of the day boats. If you call ahead they can hook you up with a DM or you might be able to buddy up with someone on the boat. I wouldn't worry about it too much. If you've got experience in a 7mm suit already then the rest is just working with the cold and the dark. Diving in California is very different but it's every bit as good as diving tropically, IMO.
Just for clarification - I've done most of my 225+ dives right here in SoCal, and I consider it to be world-class diving. I've also dived in Cozumel, Bali, Bora Bora, Belize, Galapagos, and Costa Rica, among other warm-water destinations, so I think I've been enough places to make a reasonable comparison.
My description was not meant to be "harsh", but to be realistic. Too many people who've never done a dive that wasn't led by a DM in crystal-clear, bathtub-warm water have come here and been disappointed or, worse, gotten themselves in trouble by being unprepared for the conditions and expectations around here. Not too long ago we had a rash of tourist divers who lost their lives entangled in kelp, with much speculation that it was due to their lack of preparedness in how to deal with kelp, cold water, low vis, and lack of DM guides.
So if I sound like I'm being critical of our wonderful local diving, please understand that it was only to ensure that the same thing doesn't happen to another unprepared tourist. Knowledge is power. Conditions ARE more challenging around here than many other popular dive destinations, and it certainly behooves anyone who plans to dive out here, to know what to expect.
All that being said, as much as I love warm-water diving (and I do it whenever I can), I am absolutely enamored of our spectacular underwater world out here. There is nothing more magical to me than finning through a kelp forest, feeling like I am actually flying through woods on land, weaving in and out of the "trees" like I'm Superman. I love seeing bat-rays, I love watching our ubiquitous sea lions frolicking, I REALLY love bug-hunting (or, when out-of-season, taking photos of bugs in holes), and we have some of the coolest nudis around. I don't mind the low vis, unless of course it gets down to zero. I'm happy to find myself a nice rock reef and take macro shots of all the cool little stuff there. And on good vis days, the scenery down there just can't be beat, especially when you see the sun dappling down through the kelp fronds and lighting up a swirling school of bait fish.
If someone is willing to do a little homework so they are prepared for how things are around here, then they will find some of the best diving on the planet.
