If you can't find a dive boat from the mainland, there is another option use the following site to schedule a quick trip on one of the large express boats to Catalina
http://www.catalinaexpress.com/public_html/sched_next.htm
You'll get to Avalon a bit early (before diveshops open), so take some time to eat breakfast and walk around a bit. When either of the Avalon dive shops opens (ask anyone for quick walking directions) and go rent whatever gear you didn't bring. Then walk over to Casino Point Dive Park (there's a Jacques Cousteau plaque in relatively shallow water and decent sized wreck called the Su Jac (spelling). The area is roped off and ranges from 5 feet of water to 110 feet.
Depending on weather and season (usually weekends unfortunately) the Scuba Luv dive shop also opens a large trailer right on at the point. It's incredibly easy diving, just don your gear and walk right down the stairs into the water, vis for the area has been exceptional in the last two weeks, and the water is filled with baby squid. If you are able to dive outside the park and have your game license it's lobster season and remember California has the non-pinching kind
would make a great dinner gift for whomever you are houseguests of. But remember you can't take game in the park, so this will work mainly if you find a regular dive boat, don't forget your game license with ocean enhancment sticker (you'd only need a $10.00 1-day version). One last thing it did rain yesterday, that can change vis.
If you decide not to dive catalina ... there is plenty of mid week action through local dive clubs and you can almost always find a partner at Laguna Sea Sports (lagunaseasports.com) off hwy 1 in Laguna Beach ... less than 1 hour drive south of L.A. or north of San Deigo. But there is also great local action outside of those cities as well. Mission Bay in San Deigo has kelp and wreck dive opportunities mid-week also.
Here is the main california dive boat page:
http://www.californiadiveboats.com/
Also a popular webpage all things california diving with shop listings:
http://diver.net/
Here are some midweek boat schedules and sites to other channel islands
http://www.peaceboat.com/cal2005.htm phone: 866-984-2025
http://www.explorerdiveboat.com/calendar.htm
http://calboatdiving.com/calendar/calendar.htm
Also Sport Chalet has midweek charters you can schedule directly through their stores use this link to locate store closest to your destination:
http://www.sportchalet.com/corp/index.jsp
Do you have an exact date and time you are available?