Water pushes harder on the bottom of an object than on its top, because pressure grows with depth (p = ρ·g·h). The difference is an upward force equal to the weight of the water the object pushes aside. A floating object sinks until the water it displaces weighs as much as it does. A steel ship floats because its hull displaces a lot of water. One cubic metre of water weighs 9.8 kN.