Draft is the distance vertically from the water surface to the lowest point on the boat. For sailboats and displacement powerboats, the lowest point is normally the bottom of the keel; on planing boats it is usually the propeller blade tips. A vessel floats because water exerts a buoyant force on the hull. This force exactly equals the weight of the water that is displaced by the hull. In order to float, the vessel must weigh no more than the water it displaces - this combined with total surface area is what dictates draft. 
- Chapman, Charles F. (2013). Chapman Piloting & Seamanship (67th ed.). pp. 19.