A Yacht is a large pleasure boat. There's no exact definition, but you wouldn't call a 15 ft motor boat a yacht or a 40 ft work boat. Any well maintained 30 ft plus boat that's not used for commercial purposes could reasonably be called a yacht.

Edited to add: The structure in the article is in my opinion, not a boat: Boats have a means of propulsion, (sails, oars, motor) this floating school is a barge.