A dream about a sea shell can mean you desire to visit the beach. Your brain may have triggered this image when you thought about the beach/ sea/ sea creatures, during your day.
Dreaming of seeing a sea shell, is different from dreaming you are a sea shell. This is highly unusual as it is a fantasy and may mean the dreamer is detached from their human self. There might be identity confusion if a dreamer sees imagery of themselves as non-humans.
A sea shell is the home of its organism inside, who hides most of the time. Perhaps you are being compared to this shy creature because you hide inside your shell.