I was going to say the same. 1/8" ply stitched with wire, taped with duct tape, and painted with latex would be a sturdier, and easier to work with material than cardboard, and then you can have a race, like the "minimal regatta" that I entered, where you get one sheet of plywood, two 2/4's and some duct tape, and you had to build a boat. One entry was a very small kayak with a tiny little outrigger. (The one sheet of plywood limit made the boats rather small. You could build a very good kayak with two sheets.)

You could have prizes for best artwork, fastest boat, and for the one that didn't quite make it... fastest sinking! It makes it fun for even those that didn't win.

Good luck,
Malcolm