The seashore is the land that borders an ocean or sea. She sells seashells on the seashore because that’s where the shells are, on the beach. Now say the whole thing three times fast.
You can call the seashore the coast, the beach, or even just the shore. It's the area right next to the sea, and it can be rocky and dramatic or soft and sandy. Sometimes scientists use this word to mean the specific area that's covered with water at high tide but uncovered at low tide. This area is also known as the intertidal zone.