I tried to make a boat that was small enough to fit in the back of any car, for I don't have a boat trailer. It was modeled off of the boat in this video: http://www.youtube.com/watch?v=6V9oTSprHrs
Basically, I made the boat just large enough to fit me once I sit inside of it. It displaces enough water to do so, however, I sit quite low in the water, so any sudden movements (or even any type of movement) will probably cause water to splash inside. I caulked and screwed each of the pieces of wood to each other, so it is quite stable. The acrylic paint went on nicely and after two coats I don't think any water will be soaking the plywood. (As quickly as it would have)
Improvements: There is literally no room to move; I would probably fall out. Next time I think a bigger boat would be better to build, or some sort of removable ballasts on either side to stabilize it. The propulsion system was also lacking, but a kayak paddle, or piece of wood, should do.
Crazy Idea: If I did make a bigger boat, I could probably make a sail out of PVC and old tent parts and try to sail it. (But not at my current proving grounds.) How knows, maybe even a cabin could be built relatively easily.