Swimming is nice around summertime at all beaches.
(Swimming without waves is possible at Mother's beaches in Marina Del Rey, Belmont Shore, Redondo Beach and Huntington Harbour).
Beaches Water Temps are often about 1-2 degrees warmer along northern Orange County Coast (like Seal Beach, Huntington Beach, and Newport Beach) and nearby Belmont Shore to San Pedro.
July through October, water is warmest (above 65, sometimes about 70), especially after several days of high air temperatures.
November-December water temps are usually about 60.
January-March average about 57-58. Wetsuits would be advised in the winter.
ikitesurf.com posts water temperatures, also Water Temp LA & Santa Monica.
LA and Orange County beaches do experience rip currents occasionally. (When present, rip currents pull you out to sea, but are not known to pull you under water, generally. Always check with the lifeguard at the beach, swim with partners, and watch kids carefully.) Jellyfish are occasionally known to sting, though they are not considered poisonous for most people.
After a rain, don't swim at most beaches, because storm drains bring contaminants with bacteria. Check WatchTheWater.org for Beach Conditions.