Northern California has most of the lakes that are open for seaplane operations. Lake Berryessa north of Napa and Clear Lake east of Ukiah are used often by seaplane pilots. Another great seaplane recreation area is the Sacramento/San Joaquin River delta, which has several hundred miles of waterways that are suitable for seaplane operations. Lost isle on the San Joaquin River is a great “out-of-the-way” recreational area. They have live entertainment in the summer months as well as food and drinks, lots of heavy boat traffic so check out your landing site carefully.
There are several lakes along the eastern side of the San Joaquin valley suitable for seaplane operations but once you get east of the Tehachapi Mountains or south of the Grapevine pass, landing sites are few. Those available in southern California are the San Pedro outer harbor, south end of San Diego Bay, and the Colorado River system. Keep in mind that the National Park Service prohibits taxiing under power into the marina of Lake Davis
With the above in mind, there are numerous places where seaplanes can operate and have a great time.