Although clay is not a common boat-building material, it does have something in common with many of them; it is more dense than water. A solid chunk of clay, just like a chunk of steel, fiberglass, aluminum or cement, will sink in water. In order to make clay float, you have to shape it into something wide enough and roughly bowl-shaped so that it can displace enough water to compensate for its mass, just like a real boat.
Things You'll Need
Oven or kiln
Choose the right clay. Sculpting clay is nice because it is oil-based. It won't absorb water or dissolve in it. The downside is that it won't hold its shape permanently. Water-based clay will, but you will need to bake or fire it before putting it in water, since the water will dissolve the clay.
Pat the clay into an even, rectangular flat sheet. The thinner it is, the more buoyant your boat will be.
Fold the ends of the boat together to make an enclosed box. It could be rectangular or shaped like a canoe, as long as it has sides that are higher than the bottom. Pinch the ends together to get a tight seal.
Round the bottom of the boat with your fingers and finish shaping it into whatever design you want. Look over the whole boat to see if there are any and smooth over any holes or thin spots. You may need to use small chunks of clay to patch any holes in the hull.
Bake or fire water-based clay. Some water-based clays can be cooked in a normal oven, while others need to be kiln-fired.
Float the boat in water. If it tips over, pour sand into the middle of the bottom of the boat until it stays upright.