Diver_Jan:
Hi Kris,
I would suggest diving Laguna Beach. There is a great spot there to see numerous sea lions at Seal Rock.
All the best,
I ditto what Jan said. On a weekday, there aren't too many dive boats going out from the mainland, so with beach diving, Seal Rock (at Crescent Beach) is gonna be your best chance at finding a Sea Lion and seeing Kelp. Of course, you'll have to worry about waves - have you ever done a beach enterance with waves before? If not, make sure to hook up with a local to show you the ropes (I would volunteer, but I'm still not good at waves either. If they're more than a foot or 2, then I don't dive). Laguna Sea Sports,
http://www.lagunaseasports.com/, is pretty close, so you might want to call them to see if they have someone to show you around.
Or, depending on your budget, you could take the Catalina Express over to Catalina and dive at Casino Point (or, really depending on your budget, Avalon has dive boats that go out during the week, so you could do those, but that gets pricey to take the Express over, then pay for the other boat). Catalina diving is really the nicest we have here, and the dive park will definitely give you great kelp adventures. I've never seen a sea lion in the park, though, but I do know that they go through there. Just never been lucky enough myself.
The other great spot for sea lions is Santa Barbara Island - it's known for its rookery. But, that's about a 5 hour boat ride from LA, so most boats board the night before to head out, and I don't know of any that will go on a weekday.
I hope that helps - never dove in San Diego, so I don't have advice to give you for there. If you choose any of the above, flying into LAX, Orange County, or even Ontario, CA (which is usually the cheapest to fly to, and will be the closest to Palm Springs to drive back to) will all be good options.