A paper boat, in fact, floats pretty well. Fabricating one is easy, similar to making a party hat. Should you combine both at one event, afterward you can toss the soggy boats and hats in with the paper recycling. (Which is much better than seeing silly, bright store-bought party hats with that ludicrous under-the-chin elastic in the trash the next morning and wondering what your life has come to.)
This site has instructions for simple floating origami and also puzzles and mathematic curiosities (in English and German):
http://www.mathematische-basteleien.de/paper_ship.htm
Pix and other images 