As the spell states.. it is especially harmful to water... and as fire as the opposite element, they would acording to the describtion not take much damage or none at all.
You want to know how Fire deal bludgeoning damamge is... MAGIC
its a magic world with magic creatures...
but remember. these elemental ideas are made from the early greeks theory of how the world was build. its PURELY made of that specific element (with earth being a slight execption here)
Water is WATER, Air is the air .. gasses. Fire is FIRE and Earth is Solid matter.
So....
once more
Water is WATER, Air is the air .. gasses. Fire is FIRE and Earth is Solid matter.
now someone started the argument fire has water.. that equal to Fire is water Which doesn't make sense in any way, and it is certainly NOT the way Wotc made it to work.
the d&d system is made on counters.. there is a counter to everything.
Good - Evil, Law - Chaos, Devil - Celestial, Living - Dead, Fire - Water, Air - Earth
So with fire-water being a counter, a spell particularly good or directed at the particular thing, would be less effective or wouldn't work at all agaisnt the other.
Examples could be "holy damage" or "Unholy damamge" "Fire Damage" etc, Then there is describes in the book of vile darkness and book of Exalted deeds are damage type each creatures or opposet types.
And in that way, the Horrid wilting works too, --> Godo vs Water.. Bad vs Fire
(How you handle this is a matter of Flavor)
now thats the magic part of view...
now the physical and scientific way of explanation...
Fire is Warmth Energy, its a visionary to see release of energy.
Release of energy is "noticed" in more than one way.. usually theres some sound too, thought fire is soundless.
Fire comes from release of Carbon, i.e. the stoff living matter is build of.
so if you burn wood, you release its carbon you are able to see the flames, but it is only atoms moving at an exeptional rate that produce this light.
You could indeed state that fire in some way is the same as air, just air moving at a faster rate, - hence the heat. Its free Atoms moving around eachother without being bound together by anything, water on the other hand is a Liquid, with its bound atoms forming molecules.
So the diffinition of Liquid and of Fire are diffrend hence they are NOT the same.
And a spell working on Liquid would in that way nOT work on fire, as they contain no liquid at all.