The problem with throwing water on any oil fire is that it - in the case of petrol - just distributes the fire without putting it out (it's the vapour that is burning), and - in the case of say cooking oil - results in an explosion of steam dissociation that burns your house down. read more
The fire at petrol pumps is mainly due to oil leakages. If we use water to extinguish the fire it does not workout because water do not mix with oil and settles at bottom. Further due to intense heat it just evaporates without extinguishing the fire. read more
Your answer: We should not throw water in the petrol fire because oil, petrol etc are a bit weightless than water so it continues to flow over the water. read more