Why Boats float?

A boat stays afloat because its weight is equal to that of the water it displaces. The material of the boat itself may be heavier than water (per volume), but it forms only the outer layer. Inside it is air, which is negligible in weight. But it does add to the volume. The central term here is density, which is mass ('weight') per volume. The mass of the boat (plus contents) as a whole has to be divided by the volume below the waterline. If the boat floats, then that is equal to the density of water (1 kg/l). To the water it is as if there is water there because the average density is the same. If weight is added to the boat, the volume below the waterline will have to increase too, to keep the mass/weight balance equal, so the boat sinks a little to compensate.