There are no restrictions on what you could build the boat with. But there are choices in materials that could greatly change the outcome of your boat for example, a boat made out of rocks will not do nearly as well as a boat made out of wood or even paper. The materials that are used are also affected on how available the materials are for you. Also how you put the boat together for example if you build a wooden boat, even if it is made out of wood if it has giant holes in it the boat will not float.