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Density and Bouyancy
There are two key terms to know when discussing the ability of objects to float in water. The first is density, which is an object's mass per unit of volume. Bouyancy is the second; it's the ability of an object to remain afloat.
Misconceptions
Many people believe that, because some large objects--such as floating piers or boats--have no problem staying afloat in salt water, that any rock of a certain weight will be able to float. That's not true because of the concepts of bouyancy and water displacement.
Salt Water vs. Freshwater
Salt water has an average density of 1.022. So, although it is more dense than fresh water, it doesn't give objects such as rocks the ability to float.
Expert Insight
Most rocks have a density that is greater than both freshwater and salt water. This is the main reason why a rock can't float in salt water.
Considerations
Pumice is know to have a greater buoyancy in water because of its low density. At a density of .0023, it's one of the least dense rocks.