I live in Florida and know a couple of people who have saltwater ponds. The saltwater comes from comes from the groundwater. I think they have to go deeper for the saltwater, but I will make sure to ask. I don't know if you can do it in California, but because Florida is low-lying and surrounded by saltwater, it can be pumped up using an artisan well (keep in mind I know nothing of ponds). One of my dad's friends has a saltwater pond thats about 100 feet by 50 feet and about 15 feet deep, about 560,000 gallons. He keeps native fish in there that can tolerate brackishwater (in case it rains) and its really amazing to see, especially when they get fed. Since he works as a charter captain and always has leftover fish, he feeds the scraps to the fish.
From what I saw, in his pond, he has a couple of tarpon, a cobia (thats about 35lbs), some jacks, snook, black drums, redfish, grouper, and is thinking about adding some bull sharks. All of the fish in the pond were caught by him and he transported them using a huge container. He also adds live bait to the pond in case he goes away. Even though they are native Florida fish, they act just like aquarium fish and beg for food.
So a saltwater pond can be done without using bags of salt, you just have to live in the right place