Adult harbor seals eat squid, crustaceans, molluscs, and a variety of fish; including, rockfish, herring, flounder, salmon, hake, and sand lance.

A harbor seal's diet varies seasonally and regionally and often is subject to local prey availability.

Adult harbor seals eat 5% to 6% of their body weight per day, about 4.5 to 8.2 kg (10-18 lb.).

Harbor seals don't chew their food. They swallow their food whole or tear it into chunks. With their back molars, they crush shells and crustaceans.

In dark or low light conditions, a harbor seal uses its sensitive vibrissae to find food.

Harbor seals generally obtain the water they need from their food. If food intake is decreased, the metabolic breakdown of fat produces water. The metabolism of 0.45 kg (1 lb.) of fat produces 0.64 kg (1.4 lb.) of water.