You may need to experiment with several designs until you create one that works the way you want it to.
I would like your photo submission to show your creation in water, either floating, or perhaps in its sunken state, whichever you choose.
If you would like to take this a step further, you can place heavier objects like marbles or coins in your boat to see how many it can hold before it sinks. I'd love to hear all the details and so would everyone else!
Here is some great information about buoyancy to get you started.
Here you can see how other kids from around the world responded to this challenge.