California sea lions are predators that obtain all their food from the sea. They feed on more than 50 species of fishes and cephalopods, feeding primarily on squids, octopuses, hake, northern anchovy, opaleye, and herring.

They are opportunistic predators of salmon, lamprey, and some bottom-dwelling fishes. California sea lions on the Washington coast often take advantage of the winter run of steelhead salmon; the sea lions wait by the mouth of the Lake Washington drainage and consume large quantities of the salmon.

Based on records of animals at SeaWorld, adult California sea lions eat about 5% to 8% of their body weight per day (6.8-18.2 kg or 15-40 lb.).

A Galápagos sea lion may spend an average of 15.7 hours foraging at sea. A foraging trip may entail 85 to 198 dives.

A sea lion may use its sensitive vibrissae to explore and locate food.

California sea lions don't chew their food. They swallow it whole or tear it into chunks.

California sea lions generally obtain the water they need from their food. Most research indicates that California sea lions don't drink water, though males have been observed apparently drinking seawater while fasting.

During the breeding season, adult males fast when defending their territories. Leaving their territories to feed would necessitate re-establishing territorial boundaries and would result in lost mating opportunities. This fasting usually lasts a few weeks.

Scientists have found stones in the stomachs of various species of sea lions, including California sea lions. One specimen was found to have 27.2 kg (60 lb.) of stones in its stomach. Experts don't know why sea lions swallow stones. Some theories include the following: adding extra weight for ballast while swimming, helping to stop irritation from intestinal parasites, and assisting in digestion. They may also swallow stones for no reason at all.