Because water has a density of about 1.0 g/ml, the object will sink as it is more dense than the water, think about it like this, the object for example, is a rock, and you throw it into a lake, it sinks to the bottom, as it is more dense than the water is, say if another object had 0.5g/ml density, it would float, as it is less dense than the water, hope this helps :)