Well, I have never built a cardboard boat but I have raced against some guys who did. We had a local business sponsor a "trash boat" race. The rules were easy, it had to be built of anything you could find just laying around that might be considered trash. The judges looked at every boat prior to letting them sail and they actually DQ'ed some. The challenge was to travel across 400 yards of lake. Being that I am a cat sailor I had to try and build a cat.
I used about 20 five gallon buckets all stacked together 10 on each side to make pontoons. Then I used boards to make crossbars and a squared 2x2 to make the mast which I set up sloop rigged using an old blue tarp for the sail. I even made daggerboards that I could pull up and a rudder that would fold up and down.
It was no Hobie but considering everyone else was paddling tubs or boxes of some kind, I smoked everyone. I think the wind added to my win. It was blowing well over 20 mph running right down the lake. The waves were all whitecapping and coming over the side of the cardboard boats and other monos.

If I had to build a cardboard boat, I think I would go with something like a kayak, don't try and make a tub like most people do, you will sink and it will not have my strength. In my mind a kayak would be more stable and it would faster. That is just my idea!