From SmashWiki, the Super Smash Bros. wiki
The Smash Skiff (フライングプレート, Furaingupurēto, Flying Plate) is a computer-controlled platform used by the Subspace Army. It is about the size of Final Destination and appears to be used as a transport, although its lack of rails or internal storage space is curious.
In The Swamp and Outside the Ancient Ruins, the Smash Skiff was seen carrying a captured Donkey Kong to the Isle of Ancients. Diddy Kong, Captain Falcon, and Olimar ambushed the troops on it, freed DK, and rode the Skiff into the Subspace Bomb Factory as they fought enemies along the way inside the base to the Ancient Minister.