Our neighborhood picnic featured a cardboard boat race that drew six entries. All boats were constructed on the spot from cardboard, duct tape and two styrofoam noodles. I thought they would turn soggy and sink in seconds, but most held up pretty well. Stability was the main problem. The lower boat in the first photo capsized quickly, but the simple sled design of the upper boat held up great.
The bottom boat in the second photo was even narrower and capsized almost immediately. The upper boat did well, except the sides were too tall for the kids to effectively paddle.