Why is the boat in the harbor a fragment?

The phrase "the boat in the harbor" is a fragment because it lacks a complete thought; it does not contain a subject-verb combination that expresses a complete idea. Instead, it presents a noun phrase that describes a boat's location without indicating what is happening with it. For a sentence to be complete, it must convey an action or state of being. In this case, additional information is needed to form a full sentence.