Go to a home improvement store like The Home Depot and ask for a large box like the refrigerator box.
Open both ends of the box and lay it on its narrower side. Measure the longer side and using the exacto knife score the wider sides at A and B. Score the shorter side at its half ( C ) from the outside and 3 inches on its both sides from the inside ( D and E ). D, C, and E will become the "spine" of the boat to keep the bottom rigid. Because of the spine, A-F and B-G will be shorter on the bottom and create an air filled and stronger sides to better handle the pressure from the water. When you are done, you can fold in the ends tape the seems and give it a polyurethane coat to keep the water soaking into the paper.
You can also roll up more cardboard and tape it across the top edges of the sides to strengthen the edges. This design should give you about an hour of fun and a spectacular sink at the end. Have fun!!!
You can also use boxes glued together to provide small compartment for each boater and stability for the boat. Long boats can buckle in the middle and needs to be braced, but the box design provides the needed stability.
