To keep your cardboard dry, don't forget to seal the edges with caulk or silicone. If water gets into your corrugations, you can have great fun watching it get drawn through the corrugation just like in a drinking straw. H ere s a bunch of other items to think about. A flat bottom is recommended. A V-shaped bottom is likely to tip over unless the V is very gentle.
Longer boats go faster, but they are harder to turn.
Boats shorter than 10 feet are difficult to steer. For height, allow about 18 inches for you to sit and paddle effectively without the edge of your boat blocking your arms. For width, figure about 18 inches for a kayak, about 23-24 inches for a canoe. Avoid oil-based stains, caulk, and glue because the oil soaks into the cardboard, may never dry, and this weakens the cardboard. Duct tape shrinks when it is painted.