Greg Chapman at the Home Built Boat Rally forum has posted a link to a clip of the Myth Busters building a boat out of duct tape. Take a look - it is hilarious and actually rather interesting.
It took 23 rolls to build the boat. Screwfix is offering six-packs of the stuff at £20.94, so that's just over eighty quid to cover a whole boat, including centreboard and sail. So a nice little rowing boat should be buildable in a weekend for a lot less. Still more expensive than polytarp though.