Plants in the sea are usually called seaweeds. Unlike plants on land, seaweeds never have flowers. Many animals eat seaweeds or other animals that live on and near seaweeds, such as crabs, fish prawns, etc. The California sea otter often wraps its body with seaweed before sleeping to stop it from moving in the ocean. One species of seaweed, called the giant kelp, can grow to over 100 meters long.
People often gather mollusks at the seashore.
Mollusks are animals which usually have a soft body that is protected by a hard shell. They usually move very slowly on a muscular footlike body part. Commonly eaten mollusks are clams, oysters, scallops, abalone. Squid, and octopus are mollusks without hard shells. At the seashore close to where I live, I often see people gathering sea-urchins, which are animals that look like balls with many long, sharp needles.