The key to building a boat out of cardboard is increasing the amount of surface area touching the water. The best cardboard boats make use of folded and interlaced cardboard designs.
First of all, the designer needs to plan out the boat design on paper. If he plans on riding in the boat, he must take into account that it will need a rather large bottom to hold the weight of a person. When he folds the cardboard for making the corners of the boat, he can try rolling it against a piece of wood to sharpen the creases. Usually, cardboard boats are held together with duct tape or rubber cement.
Multiple layers are necessary for the bottom of the boat. This reinforces the structure against the inevitable softening of cardboard when it touches water. Between the layers, the builder adds an inner layer of cardboard that is either cross hatched (like shading in a drawing) or folded many times like an accordion. The goal here is to increase the amount of air present between the boat and the water, much like using an air-filled floatie to keep a person above water. This also increases the surface area. The more cardboard that is touching the water, the less pressure per inch there will be.