Catalina Island is fun! Bring frozen peas to attract the fish!

Definitely! If you are going as far south as San Diego, there are two nice, easy snorkels in and around La Jolla: La Jolla Cove, and La Jolla Shores. The cove is kelp forest, and the shores is flat sandy bottom, so seemingly uninteresting - except for the leopard sharks. Find a restaurant called the "Marine Room", at the south end of the beach, and go offshore in relatively shallow water, and just hang out and keep your eyes open. They get over three feet long.

Just remember to shuffle your feet when you're walking through the surf. Very small stingrays. No joke - I know at least two people that have stepped on one. Bad times.

I've always heard that the Channel Islands are where you want to go. I think there are five of them; basically right off of LA.

I will second La Jolla Cove. Great place.