It is usually cool by the waters, Oxnard included. Six Flags is in Santa Clarita, about an hour plus away, and get there early, as it will get busy and the rides can take a long time to wait in. Oxnard is about 30 minutes from Santa Barbara, which has a cool small zoo, nice harbor pier, you can go on a boat for an hour there to look around. The awesome beaches of Malibu is about 40 minutes the other way, including Leo Carrillo Beach, which I like a lot to hike around.
The water at the beaches are cold. Under 60 if I am correct. But unless it rains, it should be between 60 to 75 degrees. Sometimes will get hotter to where I am inland, but not like that at the beach.