An object would most likely sink in water if its density is greater than the density of water.
But let's make some clarifications.
First, the density of water may differ. For example, salty sea water is more dense than river water.
Second, an object may be made from several different materials. Then its density is the average one: the entire mass divided by the entire volume. An example is a metal-made ship.
Third, if we think an object doesn't contain some space, but when immersed into water it holds air in this space, then this space does count against object's volume.
Consider a metal hemisphere immersed in water with the opening faced down.