There are only a few Los Angeles area beaches that allow dogs, and even fewer that allow them to run free off leash. Huntington Beach Dog park is one of the few and is a great place to cool off with you pet while enjoying the waves. The off leash portion of the beach is located between 21st and Seapoint on PCH in Huntington Beach CA and offers a well protected beach that is packed with dogs. Although not as expansive as some other southern California beaches this park has enough room to play fetch, throw a ball, and relax in the sun; assuming you don't mind the occasional wet nosed visitor.
There is limited parking just west of PCH across from the oil fields. It is all metered, which can be a hassel, so remember to bring some quarters. We lucked out when there and someone pulled out right in front of us but if it's a nice day you may be better off parking a little ways away.
The day we were there there were literally hundreds of dogs playing around. The atmosphere is fairly relaxed and we didn't see any signs of aggressive dogs, always a concern at a public dog area. After setting up are spot and checking out the scene we let Finn off leash and he immediatly burst towards the water jumping over smaller dogs and making immediate friends with a few kids who were kicking a soccer ball.
My one complaint about the beach is the surf. Waves break very close to shore so it is dificult even for big dogs to really do any swimming. That being said for the ability to let you dog run free and discover what a wave is this park is one my favorite places we've gone with Finnegan. For a more secluded on leash park you can try Malibu's Leo Carillo State Park or just a few miles from Huntington is the Long Beach off-leash dog zone(a smaller but still off leash park that is the only one in L.A. county). Don't forget to dry your dogs ears to avoid ear infections after the trip, and enjoy your day at the beach!