Having lived up until 1 year ago in beautiful Southern California (San Diego), let me see if I can help you.
Crowds at beaches - not bad unless it is the weekend (schools back in session right after labor day except for collages). If the weather is good (as it usually is, then the crowds will be found in La Jolla and DelMar. If you are new to SoCal beaches, remember that Pacific ocean water is much cooler then Atlantic ocean. A warm day the water is 72 to maybe 74.
Catalina - You have to go north to catch the Catalina Express from San Pedro, Long Beach or Dana Point. Dana Point is closest to San Diego. Takes about 1 hour 30 mins.
Aside from beaches, Wild Animal Park, Famous San Diego Zoo and Sea World, don town San Diego and Coronado Island are great areas to visit along with Balboa Park. Prospect Street in LaJolla has nice shopping and resturants over looking the ocean. In DelMar, Jake's is excellent and so is the view.
Hope this helps.