If gravity is the force which pulls objects towards the middle of earth, buoyancy is an upward force which pushes the object on water. The amount of buoyancy matters on the amount of water displaced. For an example, if there's a rock and a bowl full of water and we put the rock on water, there would be water spilled out of the bowl. The amount of the water is the amount of buoyancy which the rock receives. From this fact, if we put an object on water, we could know if that matter is either positively buoyant, neutrally buoyant, or negatively buoyant. Firstly, if the object receives more amount of buoyancy than gravity, it is positively buoyant which makes the boat float. Secondly, if the object is receiving the same amount of buoyancy and gravity, it is neutrally buoyant which makes the boat neither float or sink. Thirdly, if the object receives more amount of gravity than buoyancy, it is negatively buoyant which makes the boat sink.