The Catalina Express can get you to Avalon, departing out of San Pedro, Long Beach, Newport, or Dana Point.
You can get to Two Harbors only from San Pedro.
The boat ride will run you in the ballpark of $30 each R/T plus parking ($10/day?)
Figure between $60 and $120 for a decent hotel in Avalon, or you can get a cabin at Two Harbors for around $30 or so... this is a stripped cabin with only a mattress and electric oil heater. Pay showers. The problem at Two Harbors is the good diving is either by boat, or you can rent a kayak if you feel like the cruise to Cherry Cove/Lion Head (really cool shallow dive).
Avalon has the underwater park out by the casino with a good variety of scenery... nice kelp reef, not unlike what you have locally, and there's a few wrecks down around 50-80ft. If you can get permission from the harbormaster and boat traffic is light, you can hit the Valiant, which is outside of the dive park about 4 or 5 moorings out.
You can get fills at the dive park this time of year, but you have to schlep everything into town to get nitrox (for $16 or $18, and the trailer will not fill a nitrox-banded tank with air).
The Catalina Express is not as diver-friendly as it used to be. You are allowed ONE checked bag, plus one tank per diver.
Divers must WEAR their weight belts while onboard. I don't remember if they allow a carry-on in addition to the checked bag and tank or not. It's always nice to have a non-diver along for the ride to provide additional suitcase space.
Personally, when I take the express I've already loaded my dive gear and 3 tanks on my buddy's boat... he meets me at the dock and takes me back (he'll spend the week).
Good move on planning the 22/23... the dive park is a popular training area, and weekends are a real zoo with open water classes kicking up the bottom
Hook up with Dr. Bill on the board here and he can show you the ropes. He lives in Avalon.
Oh... temperatures. Tough call on that one. I dive a drysuit any time it's too cold for my 3mil tropical suit. I used to dive in a 5mil fullsuit with a 5mil hooded vest and was never cold, nor was I ever too hot. Surface temps in the summer can get up as high as 70, but we're just coming off of our "June Gloom" time and temps are just beginning to climb into the high 90s out here in Chino/Pomona. Seems to me that at 100ft, it's never been warmer than 60, so I'd say you would probably be safe with the 7mil suits. It certainly won't be shorty weather.
According to the chart here (based on an average from 1992-2001)
http://www.saintbrendan.com/cdnfeb03/EZdivingCatTemp.html
You should be looking at surface temps around 68, 15ft temps around 66, 60ft temps around 63, and 100ft temps around 58