In Aristotle’s Metaphysics there is a classic analogy which we can appropriate to shed a little light on your question. Aristotle considers a bronze statue and draws a distinction between the ‘matter’ and the ‘form’ of the statue. The statue’s matter, he says, is the bronze of which it is made. The statue’s form is the arrangement of the matter, the bronze, that makes it a statue. The statue, then, is what Aristotle calls a matter-form composite.
Our knowledge of the world has advanced greatly since Aristotle; we can now describe the statue in many ways that he could not. Aristotle’s insight was that any such description can be separated into form and matter. I submit to you that further, these descriptions themselves form a sort of chain of descriptions. A given description’s form-matter composite acts as matter for the description directly above it in the chain, while the given description’s matter acts as form-matter composite for the description directly below it. Schematically it would look something like the below. Let Dj be the j(th) description with D1 being the lowest and D4 being the highest. That which plays the role of matter in a given description is denoted DjM, and that which plays the role of form is denoted DjF.
D4F (top of chain)
D3M + D3F = D4M
D3F
D2M + D2F = D3M
D2F
D1M + D1F = D2M
D1M, D1F (bottom of chain)
Using your example of water I’ll explain how this works.
The lowest level of description, D1, has form and matter. The matter, D1M, is subatomic particles – protons, neutrons, electrons. The form, D1F, is the organization of physical laws that bind them together into an atom, like hydrogen or oxygen. The next description’s matter, D2M, is the atom itself – the composite of D1M and D1F denoted D1M + D1F. D2F is then the atomic forces that bind the hydrogen and oxygen together into a water molecule. D3M then is the matter-form composite of D2M and D2F, the hydrogen and oxygen together with the forces that bring them together. D3F is the relation of water molecules to many other water molecules through polarity interactions and van der Waals forces. D4M is the composite of D3M and D3F, that is, ordinary water. D4F is a ‘top-level’ description of ordinary water’s behavior from the perspective of classical fluid mechanics.
One of a physicist’s most important tasks is to establish links of the form D1M + D1F = D2M. He must show that the properties ascribed to the matter-object in D2M correspond to the properties expected of the matter-form composite D1M + D1F. In this case, the physicist must demonstrate that subatomic particles described in D1M subjected to organization by subatomic forces described in D1F result in the observed properties of the hydrogen and oxygen atoms described in D2M. Therefore, as a physicist, I can naturally see the world in the way I’ve described above. I don’t know how well it extends to other subjects, so hopefully people will wrap their minds around it and see how far this “description ladder” will take us.
In fact, a ladder might be a good metaphor for the relation between descriptions that I’m talking about. The rungs would be the matter, since matter is what people interact with or “step on” everyday, while the side pieces that hold the rungs in place would be the form. Each ladder step would then be a description, composed of rung (matter) and sidepieces (form). The next highest rung on the ladder depends directly on those below (although this doesn’t quite get across the ‘sum’ idea above which I think is very useful).