Funny...one of my favorite pastimes as a new yorker was collecting New York esoterica...a fun related one(relevant to my "for the most part" parenthetical in my original post) is the fact that Manhattan is an island and a borough, but part of the island is actually connected to the mainland in the Bronx.
Marble hill was once fully a part of the northernmost point of Manhattan island, but a canal was cut south of it which turned Marble hill into an actual island all by itself. Later, the waterway to the north of Marble hill was diverted into the canal, so Marble Hill became connected by land to the Bronx and separated by water from the island of Manhattan, but it is still considered a part of Manhattan borough...