The University of Washington's WOOF group (Washington Open Object Fabricators) did something we haven't seen yet: they produced a 3D printed boat that didn't sink. In fact, it worked well enough to be entered into "Denny’s Seafair Milk Carton Derby", a boat race.
Hold on, how do milk cartons fit into this scenario? It turns out that milk jugs are made from HDPE plastic, a substance often 3D printed. The team spent "weeks" collecting (and cleaning, ugh) discarded HDPE milk jugs that were ultimately chopped up and fed into their homemade 3D boat printer, adapted from a plasma cutter.
The boat looks a little rough, but we suspect they were more concerned with function over form for this experiment.
But did it work? Evidently, as they placed second in the Derby out of fourteen entrants in their category.