The scientific principle that explains why objects float or sink in water is known as Archimedes’ principle. This principle states that the buoyant force acting on an object submerged in a fluid is equal to the weight of the fluid that the object displaces. Consequently, if an object's weight is less than the weight of the displaced fluid, it will float; if it is greater, it will sink. This principle is essential in understanding fluid mechanics and applies to various fields, including engineering and oceanography.