Paper boats float based on the principle of buoyancy, which is the same principle that explains why anything floats. The boat is buoyant when the weight of the amount of water it displaces is greater than the weight of the boat itself. In effect, the water pushes up against the boat with more force than the boat pushes down. This keeps it above the surface. By understanding this basic idea, floating paper boats becomes a matter of playing with different designs and finding out which ones work.
Paper boats can be as simple as a sheet of paper folded up along the edges to create a shape like a shallow bowl, or as complex as intricately folded and constructed replicas of real ship designs. Some people enjoy making boats out of paper as pieces of artwork, and others see them as a kind of scientific experiment. The goals you have for your boats will influence your overall design, but the principals which make all paper boats float or sink are the same.
To make a functional paper boat, consider its purpose. If your boat will be decorative, you will probably want to make it taller, with a sail or cabin. Paper boats designed to support weight will work best with a much simpler construction, and may be nothing more than an empty paper hull. Either way, to keep your boat floating, it will need to be watertight and balanced.
Unless you will be propelling your paper boats through the water, they don't need to conform to any standard shape. Short and squat, low and flat, or thin and pointed boats can all float if made well. Make sure your boat has sides that come up far enough above the water line that ripples don't come over the side and fill the boat with water. You will also want to make sure that your boat floats evenly. If one side rides higher in the water than the other, the boat is more likely to capsize. This is especially important for boats which will be holding cargo.
If you plan to fill your boats with miniatures for show, or weights to test its buoyancy, having a balanced boat to star with will make your job much easier when it comes to loading the cargo on board. When you add the freight to your boat, make sure that you add it evenly, so the weight is distributed and the boat doesn't tip over. Despite all our best efforts, every paper boat that sails for long enough will eventually sink.