As cyclist said the only advantage of putting water in is to stop soot and smoke, or to lower the heat from the flame ie for a low simmer. 10% is usually the reccomended amount.
But the reason I posted is more a warning about it! I mentioned this tip to an old(very old in fact
) scout leader, he told me that they used to do this and it worked pretty effectively, usually. They stopped doing it when they realised scouts would get a bottle of meths and put 10% water in, someone would get it out of the store and put in 10% water, it would be decanted into a small bottle and 10%water would be added then some would add 10% water to the burner, by this point burn times would really increase, but the advantage was the meths seemed to last for ages!