Specific Gravity is an important aspect for understanding effects on the Fire Tetrahedron
Flammable liquids such as gasoline, diesel and several other products have a lower specific gravity than water (less dense than water). These products will float on top of water so they can continue to release vapors given the correct temperature. Even if a small spill fire was "knocked down" with plain water, fuel floats to the surface, continues to release vapors and reignite or "flashback."
While a plain water fog application may cool the flammable liquid and disrupt vapor (from a small fire), such as when a spill spreads the liquid over a small area, a flammable liquid with greater size or much depth creates a major problem.

Relative density also applies to solid objects
* Balsa wood 0.2, so it floats easily.
* Aluminium 2.7, so it sinks.
* Lead 11.35, so it sinks really fast.

Notice the Specific Gravity of Mexican Crude Oil.
It is so close to water's Specific Gravity and therefore almost nuetrally bouyant.
It still floats but wave action and currents can create difficulties in cleanup efforts.