Aha. So is there an elemental plane of cheese pizza? No? What makes water special then? Why does it get to have its own plane?
I'm not trying to be an ass, I'm just pointing out that to some people, making water, fire, etc. something other than fundamental elements is significantly hampering the game.
For example - is your Plane of Fire just a really hot place with air and lots of fire? If so, then how is it the Plane of Fire when there's so much air? What is the fire burning? There has to be a lot of non-fire stuff to burn if fire works as it does in real life. On the other hand, if pure fire is itself truly an element and exists in a natural, hot, burning state without consuming anything, then there can really be a Plane of Fire and just fire. Likewise, there can be fire elementals made of "solid fire" and the efreeti City of Brass which is made of really dense fire that looks like glowing brass. (According to this theory, fires that occur on the Prime Material Plane are "imperfect" fires and thus require air and fuel.)
The fact that it is absurd from a scientific point of view isn't really the point, as science itself is optional in DnD. There's no reason that water has to be hydrogen and oxygen to support life if the Krebs process, etc. doesn't exist and water instead supports life through a more mystical process. Your assertion that all of that has to be true is not wrong, but it presupposes biochemistry, and it's worth noting that a lot of that is in conflict with the traditional Greek elemental theory of the world (which is the basis of the planes, fire elementals running around, etc.)
It might be simpler to explain some things (you mentioned life) by using a strictly scientific reality, but it will be much harder to explain other things, such as the inner workings or reasoning of magic. Of course, if you want, you can simply say, "There is a plane of fire because it's magic! Who needs a reason?"
If that satisfies you, enjoy.