Why do objects sink in water?

Any substance with a density greater than water (1.0g/ml*), will sink in water.

Related information:

Conversely, substances with a lesser density than water, will float in water.

* Some other expressions for the density of water include 1.0 kg/l, 1.0 g/cm3, and 1,000 kg/m3. In any case, density is the ratio of mass to unit volume and remains unchanging, regardless of the units used to express it.