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.
I parked my boat at a harbor
i went to the harbor The boat anchored near the harbor.
The boat motored into the harbor. The harbor was full of sailboats.
A harbour boat is of course called a harbour boat.
Depends on your boat
Boat from Lith Harbor.
uss Arizona
by boat
By tour boat.
he got it at the harbor
USS Arizona.
You take a boat from the Pueltown Harbor