If you're only planning on doing one dive, I assume that means you plan on a beach dive, correct? If so, given the fact that you just got certified, you might want to rethink this. Beach diving in SoCal is challenging, even for experienced divers, and I would not recommend it for someone who is newly certified, not familiar with the area or conditions, and not diving with someone more experienced. Challenges include very cold water (low 50's lately), potentially high surf, low visibility (especially on beach dives), and kelp (entanglement dangers).
Can you share with us your diving experience thus far? If you have little or no beach/cold water diving, I suspect most of my fellow SoCal divers will also warn you away from this.
As Mike stated above, your best bet is a boat dive or day out at Catalina. It's unfortunate that your schedule does not permit this.