I love this idea! Years ago our Yacht Club had a cardboard boat building contest every summer. It was so much fun. Our boats had to carry two passengers and we paddled them around a race course in the river.I can give you a few tips on what works and what doesn't. First of all, the best shape is like a canoe with a pointed bow and stern so it can easily move through the water. Start with the bottom and then add the sides.Rafts don't work - they always seemed to tip over. Everyone thinks the biggest problem will be the cardboard getting wet, which it will, but the boats usually fell apart at the seams before that could happen, so how you use the tape is the big concern - use a lot and make sure to reinforce all the areas that will be the most stressed, especially the seams. If you have any access to wax coated cardboard (like milk cartons) they work great. Or you could make your own protective barrier with lotion or hair conditioner. Good luck, have fun and take lots of pictures.