A pieremagoggel is a vessel, but not a boat since it cannot have a keel. The term is sometimes used to denote a ship that was no longer considered sea-worthy. Other pieremagoggels are built for fun, rather than any practical applications.
Pieremagoggel contests score the vessels based on their creativity, and ingenuity of the propulsion mechanism, humor, and esthetics; with winners in different categories. It is not surprising that students of polytechnic schools often enter the competition. With water and canals being such a big part of Dutch culture, these competitions are very popular among contestants and spectators. I grew up watching Ter Land, ter Zee, en in de Lucht [At Land, at Sea, and in the Air], a television series which featured home-built aircraft, seacraft, and landcraft competitions.
Here you can watch a movie of a pieremagoggel competition in 1976: