The water temperatures will be in the low 70s (71-73), and some years it can go up to 75.
The best place to snorkel is in La Jolla Cove and in the area underneath the cliffs just beyond Sunny Jim Cave. Another area is the kelp forest, but you may have to swim out a bit to go there.
Another place to snorkel is at La Jolla Shores, but mostly to see the leopard, nurse and guitar sharks (harmless). They are right off the beach in about waist-to-chest-deep water, and just in some particular areas.
There are several places to rent snorkel gear in La Jolla Shores.
For seafood you can go to Crab Catcher on Prospect St. in La Jolla. Another seafood restaurant is expected to open around the time of your visit just a few stores down from the Crab Catcher. I don't remember the name. Roppongi on Prospect has Asian fusion sushi and seafood. Whisknladle on Wall St. has seafood on the menu, and so does La Jolla Shores Restaurant.