The current most common 3D printer is the filament deposition modeling (FDM) type. It's a glorified computer-controlled hot glue gun that squirts plastic through a heated nozzle, buildng up the part layer by layer. It takes plastic filament -- a thin stream of plastic wound up in a coil, which it feeds into the nozzle. This plastic filament is typically ~$25-$35 a kilogram for ABS plastic. The plastic itself is about $4.5 a pound new, however you can get it as low as $1 a pound for reground waste plastic from injection molding.

If you get the plastic new or as reground plastic, you can turn it into filament using a filament extruder. It's basically a tube with a nozzle die at one end, with a screw conveyor feeding plastic into the heated die. I bought a bunch of recycled reground plastic from a guy on ebay for $25 for 16 lbs of ABS plastic. The total amount of plastic used in the cross sections was about 4 lbs, which works out to be around $6 worth of plastic for the boat. Pretty cheap if you have the setup!