The Flat-bottomed Boat is a unit unique to the campaign of Age of Empires III: The WarChiefs. It appears in Act I: Fire's scenario Crossing the Delaware. They help the colonial militia of Nathaniel Black and George Washington cross the Delaware river.
It is called
SPCXPFlatBoat in the game files and
SHP Flat Boat in the scenario editor.
This unit cannot attack, so it is advised to avoid enemy units to prevent heavy casualties when carrying passengers to the opposite side of the river.